Genetically programmed expression of proteins containing the unnatural amino acid phenylselenocysteine

ABSTRACT

The invention relates to orthogonal pairs of tRNAs and aminoacyl-tRNA synthetases that can incorporate the unnatural amino acid phenylselenocysteine into proteins produced in eubacterial host cells such as  E. coli . The invention provides, for example but not limited to, novel orthogonal aminoacyl-tRNA synthetases, polynucleotides encoding the novel synthetase molecules, methods for identifying and making the novel synthetases, methods for producing proteins containing the unnatural amino acid phenylselenocysteine and translation systems. The invention further provides methods for producing modified proteins (e.g., lipidated proteins) through targeted modification of the phenylselenocysteine residue in a protein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and benefit of: U.S. ProvisionalPatent Appl. Ser. No. 60/783,272, filed Mar. 16, 2006; and U.S.Provisional Patent Appl. Ser. No. 60/861,456, filed Nov. 28, 2006, thecontents of which are both hereby incorporated by reference in theirentirety for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

A portion of the work described herein was supported by Grant No.GM62159 from the National Institutes of Health, and grant numbers DEFG03 00ER45812 and ER46051 from the Department of Energy. The governmenthas certain rights to this invention.

FIELD OF THE INVENTION

The invention is in the field of translation biochemistry. The inventionrelates to compositions and methods for making and using orthogonaltRNAs, orthogonal aminoacyl-tRNA synthetases, and pairs thereof, thatincorporate unnatural amino acids into proteins. The invention alsorelates to methods of producing proteins in cells using such pairs andproteins made by the methods.

BACKGROUND OF THE INVENTION

The study of protein structure and function has historically relied uponthe properties and reaction chemistries that are available using thereactive groups of the naturally occurring amino acids. Unfortunately,every known organism, from bacteria to humans, encodes the same twentycommon amino acids. These 20 amino acids comprise a surprisingly limitednumber of functional groups: nitrogen bases, carboxylic acids andamides, alcohols, and a thiol group. This limited selection of R-groupshas restricted the study of protein structure and function, where thestudies are confined by the chemical properties of the naturallyoccurring amino acids. For example, the limited number of naturallyoccurring reactive R-groups has limited the ability to make highlytargeted protein modifications to the exclusion of the other amino acidsin a protein.

Chemoselective ligation reactions involving proteins are extremelyimportant for a variety of purposes, including but not limited tostudying protein-protein interaction and cellular signaling, andgenerating novel protein therapeutics. Most selective proteinmodification reactions currently used in the art involve covalent bondformation between nucleophilic and electrophilic reaction partners thattarget naturally occurring nucleophilic residues in the protein aminoacid side chains, e.g., the reaction of α-halo ketones with histidine orcysteine side chains. Selectivity in these cases is determined by thenumber and accessibility of the nucleophilic residues in the protein.Unfortunately, naturally occurring proteins frequently contain poorlypositioned (e.g., inaccessible) reaction sites or multiple reactiontargets (e.g., lysine, histidine and cysteine residues), resulting inpoor selectivity in the modification reactions, making highly targetedprotein modification by nucleophilic/electrophilic reagents difficult.Furthermore, the sites of modification are typically limited to thenaturally occurring nucleophilic side chains of lysine, histidine orcysteine. Modification at other sites is difficult or impossible.

What is needed in the art are new strategies for incorporation ofunnatural amino acids into proteins for the purpose of modifying andstudying protein structure and function, where the unnatural amino acidshave novel reaction chemistries or other properties, e.g., biologicalproperties not found in the naturally occurring amino acids. There is aconsiderable need in the art for the creation of new strategies forprotein modification reactions that modify proteins in a highlyselective fashion, and furthermore, modify proteins under physiologicalconditions. What is needed in the art are novel methods for producingprotein modifications, where the modifications are highly specific,e.g., modifications where none of the naturally occurring amino acidsare subject to cross reactions or side reactions. Novel chemistries forhighly specific protein modification strategies find a wide variety ofapplications in the study of protein structure and function and in theproduction of therapeutic proteins.

Protein Lipidation

Protein lipidation is a key post-translational modification that isinvolved in protein localization, proper intracellular proteintrafficking and protein-protein interactions. Lipidation of proteins isfrequently required for proper biological activity. This feature iscritical for the development of some therapeutic proteins. Lipidation isalso critical in studying protein-protein interactions and cellularsignaling. Unfortunately, in vitro chemoselective ligation to producelipidated proteins using the native unlipidated form of the protein isextremely difficult, and is generally limited to modification of uniquesurface exposed cysteine residues.

The biological activities of many cellular proteins require associationwith the cell membrane, which is dependent on the post-translationmodification of cysteine by lipid residues such as farnesyl, myristoyl,and palmitoyl moieties (Chernomordik and Kozlov (2003), Annual Review ofBiochemistry 72:175-207). For example, many G-protein coupled receptorsare palmitoylated, Ras proteins are both farnesylated and palmitoylated(Chernomordik and Kozlov (2003), Annual Review of Biochemistry72:175-207). While protein farnesylation is a stable and irreversiblemodification, palmitoylation is reversible, resulting in dynamicregulation of protein function, and specific targeting to cellularmembranes (Rocks et al. (2005), Science 307(5716):1746-1752).Furthermore, γ-carboxyglutamic acid is an essential modification that isimportant for calcium-dependent membrane adhesion in the coagulationcascade (Davie et al. (1991), Biochemistry 30(43): 10363-10370).

Orthogonal Translation Systems

One strategy to overcome the limitations of a limited genetic code is toexpand the genetic code and add amino acids that have novel reactiveproperties to the biological repertoire. A general methodology has beendeveloped for the in vivo site-specific incorporation of diverseunnatural amino acids into proteins in both prokaryotic and eukaryoticorganisms. These methods rely on orthogonal protein translationcomponents that recognize a suitable selector codon to insert a desiredunnatural amino acid at a defined position during polypeptidetranslation in vivo. These methods utilize an orthogonal tRNA (O-tRNA)that recognizes a selector codon, and where a corresponding specificorthogonal aminoacyl-tRNA synthetase (an O-RS) charges the O-tRNA withthe unnatural amino acid. These components do not cross-react with anyof the endogenous tRNAs, RSs, amino acids or codons in the host organism(i.e., it must be orthogonal). The use of such orthogonal tRNA-RS pairshas made it possible to genetically encode a large number ofstructurally diverse unnatural amino acids.

The practice of using orthogonal translation systems that are suitablefor making proteins that comprise one or more unnatural amino acid isgenerally known in the art, as are the general methods for producingorthogonal translation systems. For example, see InternationalPublication Numbers WO 2002/086075, entitled “METHODS AND COMPOSITIONFOR THE PRODUCTION OF ORTHOGONAL tRNA-AMINOACYL-tRNA SYNTHETASE PAIRS;”WO 2002/085923, entitled “IN VIVO INCORPORATION OF UNNATURAL AMINOACIDS;” WO 2004/094593, entitled “EXPANDING THE EUKARYOTIC GENETICCODE;” WO 2005/019415, filed Jul. 7, 2004; WO 2005/007870, filed Jul. 7,2004; WO 2005/007624, filed Jul. 7, 2004 and WO 2006/110182, filed Oct.27, 2005, entitled “ORTHOGONAL TRANSLATION COMPONENTS FOR THE VIVOINCORPORATION OF UNNATURAL AMINO ACIDS.” Each of these applications isincorporated herein by reference in its entirety. For additionaldiscussion of orthogonal translation systems that incorporate unnaturalamino acids, and methods for their production and use, see also, Wangand Schultz, “Expanding the Genetic Code,” Chem. Commun. (Camb.) 1:1-11(2002); Wang and Schultz “Expanding the Genetic Code,” Angewandte ChemieInt. Ed., 44(1):34-66 (2005); Xie and Schultz, “An Expanding GeneticCode,” Methods 36(3):227-238 (2005); Xie and Schultz, “Adding AminoAcids to the Genetic Repertoire,” Curr. Opinion in Chemical Biology9(6):548-554 (2005); Wang et al., “Expanding the Genetic Code,” Annu.Rev. Biophys. Biomol. Struct., 35:225-249 (2006); and Xie and Schultz,“A Chemical Toolkit for Proteins—an Expanded Genetic Code,” Nat. Rev.Mol. Cell Biol., 7(10):775-782 (2006).

There is a need in the art for the development of orthogonal translationcomponents that incorporate unnatural amino acids into proteins, wherethe unnatural amino acids can be incorporated at a defined position, andwhere the unnatural amino acids have novel chemical properties thatallow the amino acid to serve as a target for specific modification(e.g., lipidation) to the exclusion of cross reactions or side reactionswith other sites in the proteins. The invention described hereinfulfills these and other needs, as will be apparent upon review of thefollowing disclosure.

SUMMARY OF THE INVENTION

The invention provides compositions and methods for incorporating theunnatural amino acid phenylselenocysteine into a growing polypeptidechain in response to a selector codon, e.g., an amber stop codon, invivo (e.g., in a host cell). These compositions include pairs oforthogonal-tRNAs (O-tRNAs) and orthogonal aminoacyl-tRNA synthetases(O-RSs) that do not interact with the host cell translation machinery.That is to say, the O-tRNA is not charged (or not charged to asignificant level) with an amino acid (natural or unnatural) by anendogenous host cell aminoacyl-tRNA synthetase. Similarly, the O-RSsprovided by the invention do not charge any endogenous tRNA with anamino acid (natural or unnatural) to a significant or in some casesdetectable level. These novel compositions permit the production oflarge quantities of proteins having translationally incorporatedunnatural amino acids. The chemical properties of thephenylselenocysteine unnatural amino acid also permit the targetedmodification of that residue to produce desired conjugations, forexample but not limited to, lipid conjugations. Such specificallymodified polypeptides can find use as therapeutics and in biomedicalresearch.

In some aspects, the invention provides translation systems. Thesesystems comprise a first orthogonal aminoacyl-tRNA synthetase (O-RS), afirst orthogonal tRNA (O-tRNA), and a first unnatural amino acid that isphenylselenocysteine, where the first O-RS preferentially aminoacylatesthe first O-tRNA with the first unnatural amino acidphenylselenocysteine. In some aspects, the O-RS preferentiallyaminoacylates the O-tRNA with said phenylselenocysteine with anefficiency that is at least 50% of the efficiency observed for atranslation system comprising that same O-tRNA, thephenylselenocysteine, and an aminoacyl-tRNA synthetase comprising theamino acid sequence of SEQ ID NO: 4, 6 or 8.

The translation systems can use components derived from a variety ofsources. In one embodiment, the O-RS used in the system can comprise theamino acid sequence of SEQ ID NOS: 4, 6 or 8, and conservative variantsof that sequence. In some embodiments, the O-tRNA is an amber suppressortRNA. In some embodiments, the O-tRNA comprises or is encoded by SEQ IDNO: 1.

In some aspects, the translation system further comprises a nucleic acidencoding a protein of interest, where the nucleic acid has at least oneselector codon that is recognized by the O-tRNA.

In some aspects, the translation system incorporates a second orthogonalpair (that is, a second O-RS and a second O-tRNA) that utilizes a secondunnatural amino acid, so that the system is now able to incorporate atleast two different unnatural amino acids at different selected sites ina polypeptide. In this dual system, the second O-RS preferentiallyaminoacylates the second O-tRNA with a second unnatural amino acid thatis different from the first unnatural amino acid, and the second O-tRNArecognizes a selector codon that is different from the selector codonrecognized by the first O-tRNA.

In some embodiments, the translation system resides in a host cell (andincludes the host cell). The host cell used in not particularly limited,as long as the O-RS and O-tRNA retain their orthogonality in their hostcell environment. The host cell can be a eubacterial cell, such as E.coli. The host cell can comprise one or more polynucleotides that encodecomponents of the translation system, including the O-RS or O-tRNA. Insome embodiments, the polynucleotide encoding the O-RS comprises anucleotide sequence of SEQ ID NO: 5, 7 or 9.

The invention also provides methods for producing proteins having one ormore unnatural amino acids at selected positions. These methods utilizethe translation systems described above. Generally, these methods startwith the step of providing a translation system comprising: (i) a firstunnatural amino acid that is phenylselenocysteine; (ii) a firstorthogonal aminoacyl-tRNA synthetase (O-RS); (iii) a first orthogonaltRNA (O-tRNA), wherein the O-RS preferentially aminoacylates the O-tRNAwith the unnatural amino acid; and, (iv) a nucleic acid encoding theprotein, where the nucleic acid comprises at least one selector codon(optionally an amber codon) that is recognized by the first O-tRNA. Themethod then incorporates the unnatural amino acid at the selectedposition in the protein during translation of the protein in response tothe selector codon, thereby producing the protein comprising theunnatural amino acid at the selected position. In some aspects of thesemethods, the O-RS preferentially aminoacylates the O-tRNA with thephenylselenocysteine with an efficiency that is at least 50% of theefficiency observed for a translation system comprising that sameO-tRNA, the phenylselenocysteine, and an aminoacyl-tRNA synthetasecomprising the amino acid sequence of SEQ ID NO: 4, 6 or 8. In someaspects, the translation system includes a nucleic acid that encodes theO-RS.

These methods can be widely applied using a variety of reagents. In someembodiments, a polynucleotide encoding the O-RS is provided. In someembodiments, the O-RS comprises an amino acid sequence of SEQ ID NO: 4,6 or 8, or conservative variants thereof. Optionally, the translationsystem used in the methods includes a nucleic acid that encodes theO-RS, for example, a nucleic acid of SEQ ID NO: 5, 7 or 9.

In some embodiments of these methods, the providing a translation systemstep comprises mutating an amino acid binding pocket of a wild-typeaminoacyl-tRNA synthetase by site-directed mutagenesis, and selecting aresulting O-RS that preferentially aminoacylates the O-tRNA with theunnatural amino acid. The selecting step can comprises positivelyselecting and negatively selecting for the O-RS from a pool of resultingaminoacyl-tRNA synthetase molecules following site-directed mutagenesis.In some embodiments, the providing step furnishes a polynucleotideencoding the O-tRNA, e.g., an O-tRNA that is an amber suppressor tRNA,or an O-tRNA that comprises or is encoded by a polynucleotide of SEQ IDNO: 1. In these methods, the providing step can also furnish a nucleicacid comprising an amber selector codon that is utilized by thetranslation system.

These methods can also be modified to incorporate more than oneunnatural amino acid into a protein. In those methods, a secondorthogonal translation system is employed in conjunction with the firsttranslation system, where the second system has different amino acid andselector codon specificities. For example, the providing step caninclude providing a second O-RS and a second O-tRNA, where the secondO-RS preferentially aminoacylates the second O-tRNA with a secondunnatural amino acid that is different from the first unnatural aminoacid, and where the second O-tRNA recognizes a selector codon in thenucleic acid that is different from the selector codon recognized by thefirst O-tRNA.

The methods for producing a protein with an unnatural amino acid canalso be conducted in the context of a host cell. In these cases, a hostcell is provided, where the host cell comprises the unnatural aminoacid, the O-RS, the O-tRNA and the nucleic acid with at least oneselector codon that encodes the protein, and where culturing the hostcell results in incorporating the unnatural amino acid. In someembodiments, the providing step comprises providing a eubacterial hostcell (e.g., E. coli). In some embodiments, the providing step includesproviding a host cell that contains a polynucleotide encoding the O-RS.For example, the polynucleotide encoding the O-RS can comprise anucleotide sequence of SEQ ID NO: 5, 7 or 9.

In some variations of these methods, the procedures further includemodification of the phenylselenocysteine following its incorporationinto a polypeptide. For example, the phenylselenocysteine can be reactedunder certain conditions that results in conversion to dehydroalanine atthe selected position. This reaction can be by oxidative elimination.The reaction can be carried out by exposing the phenylselenocysteine tohydrogen peroxide.

The invention also provides methods for producing lipidated proteins,where the lipid is conjugated at a designated selected position. Thesemethods utilize the translation systems described above. Generally,these methods start with the step of providing a translation systemcomprising: (i) a phenylselenocysteine unnatural amino acid; (ii) anorthogonal aminoacyl-tRNA synthetase (O-RS); (iii) an orthogonal tRNA(O-tRNA), where the O-RS preferentially aminoacylates the O-tRNA withthe phenylselenocysteine; and, (iv) a nucleic acid encoding the protein,where the nucleic acid comprises at least one selector codon (optionallyan amber codon) that is recognized by the O-tRNA. The method thenincorporates the phenylselenocysteine at the selected position in theprotein during translation of the protein in response to the selectorcodon. That phenylselenocysteine is then reacted to producedehydroalanine at the selected position, which is in turn itself reactedwith a lipid to produce a lipidated amino acid moiety, thereby producinga protein having a lipid at the selected position in the protein.

In some aspects of these methods, reacting the phenylselenocysteine isby oxidative elimination or exposure to hydrogen peroxide. In someaspects, the conjugated lipid reacted with the dehydroalanine can bethiopalmitic acid, farnesylmercaptan or 1-hexadecanethiol. The lipidatedamino acids thus formed are palmitoylcysteine, farnesylcysteine andS-hexadecylcysteine. Modification of the dehydroalanine is typically bya Michael Addition reaction.

The invention also provides methods for producing a protein having adehydroalanine residue at a selected position. These methods utilize thetranslation systems described above. Generally, these methods start withthe step of providing a translation system comprising: (i) aphenylselenocysteine unnatural amino acid; (ii) an orthogonalaminoacyl-tRNA synthetase (O-RS); (iii) an orthogonal tRNA (O-tRNA),where the O-RS preferentially aminoacylates the O-tRNA with thephenylselenocysteine; and, (iv) a nucleic acid encoding the protein,where the nucleic acid comprises at least one selector codon (optionallyan amber codon) that is recognized by the O-tRNA. The method thenincorporates the phenylselenocysteine at the selected position in theprotein during translation of the protein in response to the selectorcodon. That phenylselenocysteine is then reacted to producedehydroalanine at the selected position. In some aspects of thesemethods, reacting the phenylselenocysteine is by oxidative eliminationor exposure to hydrogen peroxide.

The invention also provides a variety of compositions, including nucleicacids and proteins. The nature of the composition is not particularlylimited, other than the composition comprises the specified nucleic acidor protein. The compositions of the invention can comprise any number ofadditional components of any nature.

For example, the invention provides compositions comprising O-RSpolypeptides, where the polypeptides comprise the amino acid sequence ofSEQ ID NO: 4, 6 or 8, or a conservative variant thereof. In someaspects, the conservative variant polypeptide aminoacylates a cognateorthogonal tRNA (O-tRNA) with an unnatural amino acid with an efficiencythat is at least 50% of the efficiency observed for a translation systemcomprising the O-tRNA, the unnatural amino acid, and an aminoacyl-tRNAsynthetase comprising the amino acid sequence of SEQ ID NO: 4, 6, 8 or10. The invention also provides polynucleotides that encode any of thesepolypeptides above. In some embodiments, these polynucleotides cancomprise a nucleotide sequence of SEQ ID NO: 5, 7 or 9. In someembodiments, the polypeptides are in a cell.

The invention also provides polynucleotide compositions comprising anucleotide sequence of SEQ ID NO: 5, 7 or 9. In some embodiments, theinvention provides vectors comprising the polynucleotides, e.g.,expression vectors. In some embodiments, the invention provides cellscomprising a vector described above.

DEFINITIONS

Before describing the invention in detail, it is to be understood thatthis invention is not limited to particular biological systems, whichcan, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular embodimentsonly, and is not intended to be limiting. As used in this specificationand the appended claims, the singular forms “a”, “an” and “the” includeplural referents unless the content clearly dictates otherwise. Thus,for example, reference to “a cell” includes combinations of two or morecells; reference to “a polynucleotide” includes, as a practical matter,many copies of that polynucleotide.

Unless defined herein and below in the reminder of the specification,all technical and scientific terms used herein have the same meaning ascommonly understood by one of ordinary skill in the art to which theinvention pertains.

Orthogonal: As used herein, the term “orthogonal” refers to a molecule(e.g., an orthogonal tRNA (O-tRNA) and/or an orthogonal aminoacyl-tRNAsynthetase (O-RS)) that functions with endogenous components of a cellwith reduced efficiency as compared to a corresponding molecule that isendogenous to the cell or translation system, or that fails to functionwith endogenous components of the cell. In the context of tRNAs andaminoacyl-tRNA synthetases, orthogonal refers to an inability or reducedefficiency, e.g., less than 20% efficiency, less than 10% efficiency,less than 5% efficiency, or less than 1% efficiency, of an orthogonaltRNA to function with an endogenous tRNA synthetase compared to anendogenous tRNA to function with the endogenous tRNA synthetase, or ofan orthogonal aminoacyl-tRNA synthetase to function with an endogenoustRNA compared to an endogenous tRNA synthetase to function with theendogenous tRNA. The orthogonal molecule lacks a functionally normalendogenous complementary molecule in the cell. For example, anorthogonal tRNA in a cell is aminoacylated by any endogenous RS of thecell with reduced or even zero efficiency, when compared toaminoacylation of an endogenous tRNA by the endogenous RS. In anotherexample, an orthogonal RS aminoacylates any endogenous tRNA a cell ofinterest with reduced or even zero efficiency, as compared toaminoacylation of the endogenous tRNA by an endogenous RS. A secondorthogonal molecule can be introduced into the cell that functions withthe first orthogonal molecule. For example, an orthogonal tRNA/RS pairincludes introduced complementary components that function together inthe cell with an efficiency (e.g., 45% efficiency, 50% efficiency, 60%efficiency, 70% efficiency, 75% efficiency, 80% efficiency, 90%efficiency, 95% efficiency, or 99% or more efficiency) as compared tothat of a control, e.g., a corresponding tRNA/RS endogenous pair, or anactive orthogonal pair (e.g., a tyrosyl orthogonal tRNA/RS pair).

Orthogonal tyrosyl-tRNA: As used herein, an orthogonal tyrosyl-tRNA(tyrosyl-O-tRNA) is a tRNA that is orthogonal to a translation system ofinterest, where the tRNA is: (1) identical or substantially similar to anaturally occurring tyrosyl-tRNA, (2) derived from a naturally occurringtyrosyl-tRNA by natural or artificial mutagenesis, (3) derived by anyprocess that takes a sequence of a wild-type or mutant tyrosyl-tRNAsequence of (1) or (2) into account, (4) homologous to a wild-type ormutant tyrosyl-tRNA; (5) homologous to any example tRNA that isdesignated as a substrate for a tyrosyl-tRNA synthetase in FIG. 2, or(6) a conservative variant of any example tRNA that is designated as asubstrate for a tyrosyl-tRNA synthetase in FIG. 2. The tyrosyl-tRNA canexist charged with an amino acid, or in an uncharged state. It is alsoto be understood that a “tyrosyl-O-tRNA” optionally is charged(aminoacylated) by a cognate synthetase with an amino acid other thantyrosine, respectively, e.g., with an unnatural amino acid. Indeed, itwill be appreciated that a tyrosyl-O-tRNA of the invention isadvantageously used to insert essentially any amino acid, whethernatural or unnatural, into a growing polypeptide, during translation, inresponse to a selector codon.

Orthogonal tyrosyl amino acid synthetase: As used herein, an orthogonaltyrosyl amino acid synthetase (tyrosyl-O-RS) is an enzyme thatpreferentially aminoacylates the tyrosyl-O-tRNA with an amino acid in atranslation system of interest. The amino acid that the tyrosyl-O-RSloads onto the tyrosyl-O-tRNA can be any amino acid, whether natural,unnatural or artificial, and is not limited herein. The synthetase isoptionally the same as or homologous to a naturally occurring tyrosylamino acid synthetase, or the same as or homologous to a synthetasedesignated as an O-RS in FIG. 2. For example, the O-RS can be aconservative variant of a tyrosyl-O-RS of FIG. 2, and/or can be at least50%, 60%, 70%, 80%, 90%, 95%, 98%, 99% or more identical in sequence toan O-RS of FIG. 2.

Cognate: The term “cognate” refers to components that function together,or have some aspect of specificity for each other, e.g., an orthogonaltRNA and an orthogonal aminoacyl-tRNA synthetase. The components canalso be referred to as being complementary.

Preferentially aminoacylates: As used herein in reference to orthogonaltranslation systems, an O-RS “preferentially aminoacylates” a cognateO-tRNA when the O-RS charges the O-tRNA with an amino acid moreefficiently than it charges any endogenous tRNA in an expression system.That is, when the O-tRNA and any given endogenous tRNA are present in atranslation system in approximately equal molar ratios, the O-RS willcharge the O-tRNA more frequently than it will charge the endogenoustRNA. Preferably, the relative ratio of O-tRNA charged by the O-RS toendogenous tRNA charged by the O-RS is high, preferably resulting in theO-RS charging the O-tRNA exclusively, or nearly exclusively, when theO-tRNA and endogenous tRNA are present in equal molar concentrations inthe translation system. The relative ratio between O-tRNA and endogenoustRNA that is charged by the O-RS, when the O-tRNA and O-RS are presentat equal molar concentrations, is greater than 1:1, preferably at leastabout 2:1, more preferably 5:1, still more preferably 10:1, yet morepreferably 20:1, still more preferably 50:1, yet more preferably 75:1,still more preferably 95:1, 98:1, 99:1, 100:1, 500:1, 1,000:1, 5,000:1or higher.

The O-RS “preferentially aminoacylates an O-tRNA with an unnatural aminoacid” when (a) the O-RS preferentially aminoacylates the O-tRNA comparedto an endogenous tRNA, and (b) where that aminoacylation is specific forthe unnatural amino acid, as compared to aminoacylation of the O-tRNA bythe O-RS with any natural amino acid. That is, when the unnatural andnatural amino acids are present in equal molar amounts in a translationsystem comprising the O-RS and O-tRNA, the O-RS will load the O-tRNAwith the unnatural amino acid more frequently than with the naturalamino acid. Preferably, the relative ratio of O-tRNA charged with theunnatural amino acid to O-tRNA charged with the natural amino acid ishigh. More preferably, O-RS charges the O-tRNA exclusively, or nearlyexclusively, with the unnatural amino acid. The relative ratio betweencharging of the O-tRNA with the unnatural amino acid and charging of theO-tRNA with the natural amino acid, when both the natural and unnaturalamino acids are present in the translation system in equal molarconcentrations, is greater than 1:1, preferably at least about 2:1, morepreferably 5:1, still more preferably 10:1, yet more preferably 20:1,still more preferably 50:1, yet more preferably 75:1, still morepreferably 95:1, 98:1, 99:1, 100:1, 500:1, 1,000:1, 5,000:1 or higher.

Selector codon: The term “selector codon” refers to codons recognized bythe O-tRNA in the translation process and not recognized by anendogenous tRNA. The O-tRNA anticodon loop recognizes the selector codonon the mRNA and incorporates its amino acid, e.g., an unnatural aminoacid, at this site in the polypeptide. Selector codons can include,e.g., nonsense codons, such as, stop codons, e.g., amber, ochre, andopal codons; four or more base codons; rare codons; codons derived fromnatural or unnatural base pairs and/or the like.

Suppressor tRNA: A suppressor tRNA is a tRNA that alters the reading ofa messenger RNA (mRNA) in a given translation system, typically byallowing the incorporation of an amino acid in response to a stop codon(i.e., “read-through”) during the translation of a polypeptide. In someaspects, a selector codon of the invention is a suppressor codon, e.g.,a stop codon (e.g., an amber, ocher or opal codon), a four base codon, arare codon, etc.

Suppression activity: As used herein, the term “suppression activity”refers, in general, to the ability of a tRNA (e.g., a suppressor tRNA)to allow translational read-through of a codon (e.g., a selector codonthat is an amber codon or a 4-or-more base codon) that would otherwiseresult in the termination of translation or mistranslation (e.g.,frame-shifting). Suppression activity of a suppressor tRNA can beexpressed as a percentage of translational read-through activityobserved compared to a second suppressor tRNA, or as compared to acontrol system, e.g., a control system lacking an O-RS.

The present invention provides various methods by which suppressionactivity can be quantitated. Percent suppression of a particular O-tRNAand O-RS against a selector codon (e.g., an amber codon) of interestrefers to the percentage of activity of a given expressed test marker(e.g., LacZ), that includes a selector codon, in a nucleic acid encodingthe expressed test marker, in a translation system of interest, wherethe translation system of interest includes an O-RS and an O-tRNA, ascompared to a positive control construct, where the positive controllacks the O-tRNA, the O-RS and the selector codon. Thus, for example, ifan active positive control marker construct that lacks a selector codonhas an observed activity of X in a given translation system, in unitsrelevant to the marker assay at issue, then percent suppression of atest construct comprising the selector codon is the percentage of X thatthe test marker construct displays under essentially the sameenvironmental conditions as the positive control marker was expressedunder, except that the test marker construct is expressed in atranslation system that also includes the O-tRNA and the O-RS.Typically, the translation system expressing the test marker alsoincludes an amino acid that is recognized by the O-RS and O-tRNA.Optionally, the percent suppression measurement can be refined bycomparison of the test marker to a “background” or “negative” controlmarker construct, which includes the same selector codon as the testmarker, but in a system that does not include the O-tRNA, O-RS and/orrelevant amino acid recognized by the O-tRNA and/or O-RS. This negativecontrol is useful in normalizing percent suppression measurements toaccount for background signal effects from the marker in the translationsystem of interest.

Suppression efficiency can be determined by any of a number of assaysknown in the art. For example, a β-galactosidase reporter assay can beused, e.g., a derivatived lacZ plasmid (where the construct has aselector codon n the lacZ nucleic acid sequence) is introduced intocells from an appropriate organism (e.g., an organism where theorthogonal components can be used) along with plasmid comprising anO-tRNA of the invention. A cognate synthetase can also be introduced(either as a polypeptide or a polynucleotide that encodes the cognatesynthetase when expressed). The cells are grown in media to a desireddensity, e.g., to an OD₆₀₀ of about 0.5, and β-galactosidase assays areperformed, e.g., using the BetaFluor™ β-Galactosidase Assay Kit(Novagen). Percent suppression can be calculated as the percentage ofactivity for a sample relative to a comparable control, e.g., the valueobserved from the derivatized lacZ construct, where the construct has acorresponding sense codon at desired position rather than a selectorcodon.

Translation system: The term “translation system” refers to thecomponents that incorporate an amino acid into a growing polypeptidechain (protein). Components of a translation system can include, e.g.,ribosomes, tRNAs, synthetases, mRNA and the like. The O-tRNA and/or theO-RSs of the invention can be added to or be part of an in vitro or invivo translation system, e.g., in a non-eukaryotic cell, e.g., abacterium (such as E. coli), or in a eukaryotic cell, e.g., a yeastcell, a mammalian cell, a plant cell, an algae cell, a fungus cell, aninsect cell, and/or the like.

Unnatural amino acid: As used herein, the term “unnatural amino acid”refers to any amino acid, modified amino acid, and/or amino acidanalogue, that is not one of the 20 common naturally occurring aminoacids. For example, the unnatural amino acid phenylselenocysteine (seeFIG. 1, structure 1) finds use with the invention.

Derived from: As used herein, the term “derived from” refers to acomponent that is isolated from or made using a specified molecule ororganism, or information from the specified molecule or organism. Forexample, a polypeptide that is derived from a second polypeptide caninclude an amino acid sequence that is identical or substantiallysimilar to the amino acid sequence of the second polypeptide. In thecase of polypeptides, the derived species can be obtained by, forexample, naturally occurring mutagenesis, artificial directedmutagenesis or artificial random mutagenesis. The mutagenesis used toderive polypeptides can be intentionally directed or intentionallyrandom, or a mixture of each. The mutagenesis of a polypeptide to createa different polypeptide derived from the first can be a random event(e.g., caused by polymerase infidelity) and the identification of thederived polypeptide can be made by appropriate screening methods, e.g.,as discussed herein. Mutagenesis of a polypeptide typically entailsmanipulation of the polynucleotide that encodes the polypeptide.

Positive selection or screening marker: As used herein, the term“positive selection or screening marker” refers to a marker that, whenpresent, e.g., expressed, activated or the like, results inidentification of a cell, which comprises the trait, e.g., a cell withthe positive selection marker, from those without the trait.

Negative selection or screening marker: As used herein, the term“negative selection or screening marker” refers to a marker that, whenpresent, e.g., expressed, activated, or the like, allows identificationof a cell that does not comprise a selected property or trait (e.g., ascompared to a cell that does possess the property or trait).

Reporter: As used herein, the term “reporter” refers to a component thatcan be used to identify and/or select target components of a system ofinterest. For example, a reporter can include a protein, e.g., anenzyme, that confers antibiotic resistance or sensitivity (e.g.,β-lactamase, chloramphenicol acetyltransferase (CAT), and the like), afluorescent screening marker (e.g., green fluorescent protein (e.g.,(GFP), YFP, EGFP, RFP, etc.), a luminescent marker (e.g., a fireflyluciferase protein), an affinity based screening marker, or positive ornegative selectable marker genes such as lacZ, β-gal/lacZ(β-galactosidase), ADH (alcohol dehydrogenase), his3, ura3, leu2, lys2,or the like.

Eukaryote: As used herein, the term “eukaryote” refers to organismsbelonging to the Kingdom Eucarya. Eukaryotes are generallydistinguishable from prokaryotes by their typically multicellularorganization (but not exclusively multicellular, for example, yeast),the presence of a membrane-bound nucleus and other membrane-boundorganelles, linear genetic material (i.e., linear chromosomes), theabsence of operons, the presence of introns, message capping and poly-AmRNA, and other biochemical characteristics, such as a distinguishingribosomal structure. Eukaryotic organisms include, for example, animals(e.g., mammals, insects, reptiles, birds, etc.), ciliates, plants (e.g.,monocots, dicots, algae, etc.), fungi, yeasts, flagellates,microsporidia, protists, etc.

Prokaryote: As used herein, the term “prokaryote” refers to organismsbelonging to the Kingdom Monera (also termed Procarya). Prokaryoticorganisms are generally distinguishable from eukaryotes by theirunicellular organization, asexual reproduction by budding or fission,the lack of a membrane-bound nucleus or other membrane-bound organelles,a circular chromosome, the presence of operons, the absence of introns,message capping and poly-A mRNA, and other biochemical characteristics,such as a distinguishing ribosomal structure. The Prokarya includesubkingdoms Eubacteria and Archaea (sometimes termed “Archaebacteria”).Cyanobacteria (the blue green algae) and mycoplasma are sometimes givenseparate classifications under the Kingdom Monera.

Bacteria: As used herein, the terms “bacteria” and “eubacteria” refer toprokaryotic organisms that are distinguishable from Archaea. Similarly,Archaea refers to prokaryotes that are distinguishable from eubacteria.Eubacteria and Archaea can be distinguished by a number morphologicaland biochemical criteria. For example, differences in ribosomal RNAsequences, RNA polymerase structure, the presence or absence of introns,antibiotic sensitivity, the presence or absence of cell wallpeptidoglycans adn other cell wall components, the branched versusunbranched structures of membrane lipids, and the presence/absence ofhistones and histone-like proteins are used to assign an organism toEubacteria or Archaea.

Examples of Eubacteria include Escherichia coli, Thermus thermophilus,Bacillus subtilis and Bacillus stearothermophilus. Example of Archaeainclude Methanococcus jannaschii (Mj), Methanosarcina mazei (Mm),Methanobacterium thermoautotrophicum (Mt), Methanococcus maripaludis,Methanopyrus kandleri, Halobacterium such as Haloferax volcanii andHalobacterium species NRC-1, Archaeoglobus fulgidus (Af), Pyrococcusfuriosus (Pf), Pyrococcus horikoshii (Ph), Pyrobaculum aerophilum,Pyrococcus abyssi, Sulfolobus solfataricus (Ss), Sulfolobus tokodaii,Aeuropyrum pernix (Ap), Thermoplasma acidophilum and Thermoplasmavolcanium.

Conservative variant: As used herein, the term “conservative variant,”in the context of a translation component, refers to a translationcomponent, e.g., a conservative variant O-tRNA or a conservative variantO-RS, that functionally performs similar to a base component that theconservative variant is similar to, e.g., an O-tRNA or O-RS, havingvariations in the sequence as compared to a reference O-tRNA or O-RS.For example, an O-RS, or a conservative variant of that O-RS, willaminoacylate a cognate O-tRNA with a phenylselenocysteine unnaturalamino acid. In this example, the O-RS and the conservative variant O-RSdo not have the same amino acid sequences. The conservative variant canhave, e.g., one variation, two variations, three variations, fourvariations, or five or more variations in sequence, as long as theconservative variant is still complementary to (e.g., functions with)the cognate corresponding O-tRNA or O-RS.

In some embodiments, a conservative variant O-RS comprises one or moreconservative amino acid substitutions compared to the O-RS from which itwas derived. In some embodiments, a conservative variant O-RS comprisesone or more conservative amino acid substitutions compared to the O-RSfrom which it was derived, and furthermore, retains O-RS biologicalactivity; for example, a conservative variant O-RS that retains at least10% of the biological activity of the parent O-RS molecule from which itwas derived, or alternatively, at least 20%, at least 30%, or at least40%. In some preferred embodiments, the conservative variant O-RSretains at least 50% of the biological activity of the parent O-RSmolecule from which it was derived. The conservative amino acidsubstitutions of a conservative variant O-RS can occur in any domain ofthe O-RS, including the amino acid binding pocket.

Selection or screening agent: As used herein, the term “selection orscreening agent” refers to an agent that, when present, allows forselection/screening of certain components from a population. Forexample, a selection or screening agent can be, but is not limited to,e.g., a nutrient, an antibiotic, a wavelength of light, an antibody, anexpressed polynucleotide, or the like. The selection agent can bevaried, e.g., by concentration, intensity, etc.

In response to: As used herein, the term “in response to” refers to theprocess in which an O-tRNA of the invention recognizes a selector codonand mediates the incorporation of the unnatural amino acid, which iscoupled to the tRNA, into the growing polypeptide chain.

Encode: As used herein, the term “encode” refers to any process wherebythe information in a polymeric macromolecule or sequence string is usedto direct the production of a second molecule or sequence string that isdifferent from the first molecule or sequence string. As used herein,the term is used broadly, and can have a variety of applications. Insome aspects, the term “encode” describes the process ofsemi-conservative DNA replication, where one strand of a double-strandedDNA molecule is used as a template to encode a newly synthesizedcomplementary sister strand by a DNA-dependent DNA polymerase.

In another aspect, the term “encode” refers to any process whereby theinformation in one molecule is used to direct the production of a secondmolecule that has a different chemical nature from the first molecule.For example, a DNA molecule can encode an RNA molecule (e.g., by theprocess of transcription incorporating a DNA-dependent RNA polymeraseenzyme). Also, an RNA molecule can encode a polypeptide, as in theprocess of translation. When used to describe the process oftranslation, the term “encode” also extends to the triplet codon thatencodes an amino acid. In some aspects, an RNA molecule can encode a DNAmolecule, e.g., by the process of reverse transcription incorporating anRNA-dependent DNA polymerase. In another aspect, a DNA molecule canencode a polypeptide, where it is understood that “encode” as used inthat case incorporates both the processes of transcription andtranslation.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides the chemical structures of the unnatural amino acidphenylselenocysteine (structure 1) as well as structures that can bederived from phenylselenocysteine. These structures includedehydroalanine (2), palmitoylcysteine (3), farnesylcysteine (4),S-hexadecylcysteine (5), and gamma-carboxyglutamic acid (6).

FIG. 2 provides various nucleotide and amino acid sequences finding usewith the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides solutions to the inherent limitations of using atranslation system confined by the 20 naturally occurring amino acids.The solutions include compositions and methods related to theprogrammed, site-specific biosynthetic incorporation of the unnaturalamino acid phenylselenocysteine (FIG. 1, structure 1) into proteinsusing orthogonal translation systems. The incorporation ofphenylselenocysteine into the protein can be programmed to occur at anydesired position by engineering the polynucleotide encoding the proteinof interest to contain a selector codon that signals the incorporationof the unnatural amino acid into the growing polypeptide chain.

The invention provides novel compositions including novel aminoacyl-tRNAsynthetases (O-RS) that have the ability to charge a suitable cognatesuppressor O-tRNA (e.g., the O-tRNA of SEQ ID NO: 1) withphenylselenocysteine. These O-RS are novel mutants of the Methanococcusjannaschii tyrosyl-tRNA synthetase that selectively charge the O-tRNAwith the unnatural amino acid phenylselenocysteine in bacterial hostcells. Most preferably, the orthogonal components do not cross-reactwith endogenous host components of the translational machinery of thehost cell (e.g., an E. coli cell).

The O-RS of the invention can include the O-RS of SEQ ID NOS: 4, 6 or 8.The invention also provides polynucleotides that encode these O-RSpolypeptides. The present disclosure also describes the methodology forevolution of the novel O-tRNA/O-RS pairs that function in eubacteria tosite-specifically incorporate a phenylselenocysteine unnatural aminoacid in response to selector codons.

The invention also provides methods for the highly efficient andsite-specific genetic incorporation of phenylselenocysteine (FIG. 1,structure 1) into proteins (preferably in vivo) in response to aselector codon (e.g., the amber nonsense codon, TAG). These novelmethods and compositions can be used in, for example, a bacterial hostsystem.

In some cases, the phenylselenocysteine unnatural amino acid can then bespecifically and regioselectively modified after its incorporation intoa polypeptide, as described in the present disclosure. Because of thereaction chemistry of the phenylseleno-group, proteins into which theunnatural amino acid is incorporated can be modified with extremely highselectivity. In some cases, the phenylselenocysteine unnatural aminoacid reactive group has the advantage of being completely alien to invivo systems, thereby improving reaction selectivity. In some aspects,the modification reactions can be conducted using relatively mildreaction conditions that permit both in vitro and in vivo conjugationreactions involving proteins, and preserving host cell viability and/orprotein biological activity.

In some aspects, the incorporated phenylselenocysteine moiety ismodified, and that modified product is then in turn again modified, forexample, by a conjugation reaction.

The nature of the material that is ultimately conjugated at the selectedposition in the protein (corresponding to the selector codon in the openreading frame encoding the protein) is not particularly limited, and canbe any desired entity, e.g., lipids, dyes, fluorophores, crosslinkingagents, saccharide derivatives, polymers (e.g., derivatives ofpolyethylene glycol), photocrosslinkers, cytotoxic compounds, affinitylabels, derivatives of biotin, resins, beads, a second protein orpolypeptide (or more), polynucleotide(s) (e.g., DNA, RNA, etc.), metalchelators, cofactors, fatty acids, carbohydrates, and the like.

In some aspects, to demonstrate (but not to limit) the presentinvention, the disclosure herein describes the phenylselenocysteineunnatural amino acid moiety incorporated into a model protein, forexample, myoglobin, human growth hormone and GFP. It is not intendedthat the incorporation of the phenylselenocysteine unnatural amino acidbe limited to any particular protein. It will be clear that theincorporation of phenylselenocysteine unnatural amino acid into anydesired protein of interest can be accomplished using the guidance ofthe present disclosure. Generation of proteins comprising one or morephenylselenocysteine unnatural amino acid (either alone or incombination with other different unnatural amino acids) is advantageousfor a variety of purposes, for example, for use in therapeutic proteinsand for research purposes.

Modification of Phenylselenocysteine

In some aspects, the invention provides methods for the modification ofthe phenylselenocysteine amino acid residue following its incorporationinto a polypeptide. One such modification, for example, oxidativecleavage, can beneficially convert the phenylselenocysteine amino acidresidue into the α,β-unsaturated amino acid dehydroalanine (see FIG. 1,structure 2). This dehydroalanine unnatural amino acid is also reactiveand can be subsequently modified.

It is not intended that the invention be limited to any particularmechanism (e.g., oxidative elimination) or specific reaction conditions(for example, exposure to hydrogen peroxide) for the conversion ofphenylselenocysteine to dehydroalanine. One of ordinary skill in the artwill recognize a variety of suitable alternative mechanisms and reactionconditions that find equal use with the invention for the conversion ofa phenylselenocysteine residue to dehydroalanine.

Modification of Dehydroalanine

In some aspects, the invention provides methods for the furthermodification of a dehydroalanine unnatural amino acid residue in apolypeptide. The dehydroalanine residue is reactive and can be targetedin highly specific modification reactions. These methods are especiallyuseful in forming lipid conjugates to produce lipidated proteins.

As described herein, Michael Addition reactions of the unnatural aminoacid dehydroalanine result in proteins having programmed, site-specificpost-translational modifications. For example, reaction ofdehydroalanine with a thio-lipid can generate a lipidated protein. Forexample, reaction with thiopalmitic acid results in palmitoylcysteine(see FIG. 1, structure 3), reaction with farnesylmercaptan producesfarnesylcysteine (see FIG. 1, structure 4), and reaction with malonateproduces γ-carboxyglutamic acid (see FIG. 1, structure 6). In addition,reaction with 1-hexadecanethiol results in S-hexadecylcysteine (see FIG.1, structure 5).

Although the present specification provides these examples, it is notintended that the invention be limited to any particular mechanism(e.g., a Michael Addition pathway) or specific reagents (thio-lipids) inthe modification (conjugation) of dehydroalanine. One of ordinary skillin the art will recognize a variety of suitable alternative mechanisms,reaction conditions and reagents that find equal use with the inventionfor the modification of the dehydroalanine residue. Indeed, modificationof the dehydroalanine residue is not limited to lipid conjugation, andthis conjugation mechanism can be used to conjugate any desired moietyto the polypeptide at the site of the dehydroalanine.

Orthogonal tRNA/Aminoacyl-tRNA Synthetase Technology

An understanding of the novel compositions and methods of the presentinvention requires an understanding of the activities associated withorthogonal tRNA and orthogonal aminoacyl-tRNA synthetase pairs. In orderto add additional unnatural amino acids to the genetic code, neworthogonal pairs comprising an aminoacyl-tRNA synthetase and a suitabletRNA are needed that can function efficiently in the host translationalmachinery, but that are “orthogonal” to the translation system at issue,meaning that it functions independently of the synthetases and tRNAsendogenous to the translation system. Desired characteristics of theorthogonal pair include tRNA that decode or recognize only a specificcodon, e.g., a selector codon, that is not decoded by any endogenoustRNA, and aminoacyl-tRNA synthetases that preferentially aminoacylate(or “charge”) its cognate tRNA with only one specific unnatural aminoacid. The O-tRNA is also not typically aminoacylated (or is poorlyaminoacylated, i.e., charged) by endogenous synthetases. For example, inan E. coli host system, an orthogonal pair will include anaminoacyl-tRNA synthetase that does not cross-react with any of theendogenous tRNA, e.g., which there are 40 in E. coli, and an orthogonaltRNA that is not aminoacylated by any of the endogenous synthetases,e.g., of which there are 21 in E. coli.

The general principles of orthogonal translation systems that aresuitable for making proteins that comprise one or more unnatural aminoacid are known in the art, as are the general methods for producingorthogonal translation systems. For example, see InternationalPublication Numbers WO 2002/086075, entitled “METHODS AND COMPOSITIONFOR THE PRODUCTION OF ORTHOGONAL tRNA-AMINOACYL-tRNA SYNTHETASE PAIRS;”WO 2002/085923, entitled “IN VIVO INCORPORATION OF UNNATURAL AMINOACIDS;” WO 2004/094593, entitled “EXPANDING THE EUKARYOTIC GENETICCODE;” WO 2005/019415, filed Jul. 7, 2004; WO 2005/007870, filed Jul. 7,2004; WO 2005/007624, filed Jul. 7, 2004; and WO 2006/110182, filed onOct. 27, 2005, entitled “ORTHOGONAL TRANSLATION COMPONENTS FOR THE INVIVO INCORPORATION OF UNNATURAL AMINO ACIDS.” Each of these publicationsis incorporated herein by reference in their entirety. For discussion oforthogonal translation systems that incorporate unnatural amino acids,and methods for their production and use, see also, Wang and Schultz“Expanding the Genetic Code,” Angewandte Chemie Int. Ed., 44(1):34-66(2005), Xie and Schultz, “An Expanding Genetic Code,” Methods36(3):227-238 (2005); Xie and Schultz, “Adding Amino Acids to theGenetic Repertoire,” Curr. Opinion in Chemical Biology 9(6):548-554(2005); Wang et al., “Expanding the Genetic Code,” Annu. Rev. Biophys.Biomol. Struct., 35:225-249 (2006); and Ryu and Schultz, “EfficientIncorporation of Unnatural Amino Acids into Proteins in Escherichiacoli,” Nat. Methods (4):263-265 (2006); the contents of which are eachincorporated by reference in their entirety.

Orthogonal Translation Systems

Orthogonal translation systems generally comprise cells (which can beprokaryotic cells such as E. coli) that include an orthogonal tRNA(O-tRNA), an orthogonal aminoacyl tRNA synthetase (O-RS), and anunnatural amino acid, where the O-RS aminoacylates the O-tRNA with theunnatural amino acid. An orthogonal pair of the invention can include anO-tRNA, e.g., a suppressor tRNA, a frameshift tRNA, or the like, and acognate O-RS. The orthogonal systems of the invention can typicallycomprise O-tRNA/O-RS pairs, either in the context of a host cell orwithout the host cell. In addition to multi-component systems, theinvention also provides novel individual components, for example, novelorthogonal aminoacyl-tRNA synthetase polypeptides (e.g., SEQ ID NO: 4, 6or 8), and the polynucleotides that encodes those polypeptides (e.g.,SEQ ID NO: 5, 7 or 9).

In general, when an orthogonal pair recognizes a selector codon andloads an amino acid in response to the selector codon, the orthogonalpair is said to “suppress” the selector codon. That is, a selector codonthat is not recognized by the translation system's (e.g., the cell's)endogenous machinery is not ordinarily charged, which results inblocking production of a polypeptide that would otherwise be translatedfrom the nucleic acid. In an orthogonal pair system, the O-RSaminoacylates the O-tRNA with a specific unnatural amino acid. Thecharged O-tRNA recognizes the selector codon and suppresses thetranslational block caused by the selector codon.

In some aspects, an O-tRNA of the invention recognizes a selector codonand includes at least about, e.g., a 45%, a 50%, a 60%, a 75%, a 80%, ora 90% or more suppression efficiency in the presence of a cognatesynthetase in response to a selector codon as compared to thesuppression efficiency of an O-tRNA comprising or encoded by apolynucleotide sequence as set forth in the sequence listing herein.

In some embodiments, the suppression efficiency of the O-RS and theO-tRNA together is about, e.g., 5 fold, 10 fold, 15 fold, 20 fold, or 25fold or more greater than the suppression efficiency of the O-tRNAlacking the O-RS. In some aspect, the suppression efficiency of the O-RSand the O-tRNA together is at least about, e.g., 35%, 40%, 45%, 50%,60%, 75%, 80%, or 90% or more of the suppression efficiency of anorthogonal synthetase pair as set forth in the sequence listings herein.

The host cell uses the O-tRNA/O-RS pair to incorporate the unnaturalamino acid into a growing polypeptide chain, e.g., via a nucleic acidthat comprises a polynucleotide that encodes a polypeptide of interest,where the polynucleotide comprises a selector codon that is recognizedby the O-tRNA. In certain preferred aspects, the cell can include one ormore additional O-tRNA/O-RS pairs, where the additional O-tRNA is loadedby the additional O-RS with a different unnatural amino acid. Forexample, one of the O-tRNAs can recognize a four base codon and theother O-tRNA can recognize a stop codon. Alternately, multiple differentstop codons or multiple different four base codons can be used in thesame coding nucleic acid.

As noted, in some embodiments, there exists multiple O-tRNA/O-RS pairsin a cell or other translation system, which allows incorporation ofmore than one unnatural amino acid into a polypeptide. For example, thecell can further include an additional different O-tRNA/O-RS pair and asecond unnatural amino acid, where this additional O-tRNA recognizes asecond selector codon and this additional O-RS preferentiallyaminoacylates the O-tRNA with the second unnatural amino acid. Forexample, a cell that includes an O-tRNA/O-RS pair (where the O-tRNArecognizes, e.g., an amber selector codon), can further comprise asecond orthogonal pair, where the second O-tRNA recognizes a differentselector codon, e.g., an opal codon, a four-base codon, or the like.Desirably, the different orthogonal pairs are derived from differentsources, which can facilitate recognition of different selector codons.

In certain embodiments, systems comprise a cell such as an E. coli cellthat includes an orthogonal tRNA (O-tRNA), an orthogonal aminoacyl-tRNAsynthetase (O-RS), an unnatural amino acid and a nucleic acid thatcomprises a polynucleotide that encodes a polypeptide of interest, wherethe polynucleotide comprises the selector codon that is recognized bythe O-tRNA. The translation system can also be a cell-free system, e.g.,any of a variety of commercially available “in vitro”transcription/translation systems in combination with an O-tRNA/O-RSpair and an unnatural amino acid as described herein.

The O-tRNA and/or the O-RS can be naturally occurring or can be, e.g.,derived by mutation of a naturally occurring tRNA and/or RS, e.g., bygenerating libraries of tRNAs and/or libraries of RSs, from any of avariety of organisms and/or by using any of a variety of availablemutation strategies. For example, one strategy for producing anorthogonal tRNA/aminoacyl-tRNA synthetase pair involves importing aheterologous (to the host cell) tRNA/synthetase pair from, e.g., asource other than the host cell, or multiple sources, into the hostcell. The properties of the heterologous synthetase candidate include,e.g., that it does not charge any host cell tRNA, and the properties ofthe heterologous tRNA candidate include, e.g., that it is notaminoacylated by any host cell synthetase. In addition, the heterologoustRNA is orthogonal to all host cell synthetases. A second strategy forgenerating an orthogonal pair involves generating mutant libraries fromwhich to screen and/or select an O-tRNA or O-RS. These strategies canalso be combined.

Orthogonal tRNA (O-tRNA)

An orthogonal tRNA (O-tRNA) of the invention desirably mediatesincorporation of an unnatural amino acid into a protein that is encodedby a polynucleotide that comprises a selector codon that is recognizedby the O-tRNA, e.g., in vivo or in vitro. In certain embodiments, anO-tRNA of the invention includes at least about, e.g., a 45%, a 50%, a60%, a 75%, a 80%, or a 90% or more suppression efficiency in thepresence of a cognate synthetase in response to a selector codon ascompared to an O-tRNA comprising or encoded by a polynucleotide sequenceas set forth in the O-tRNA sequences in the sequence listing herein.

Suppression efficiency can be determined by any of a number of assaysknown in the art. For example, a β-galactosidase reporter assay can beused, e.g., a derivatized lacZ plasmid (where the construct has aselector codon n the lacZ nucleic acid sequence) is introduced intocells from an appropriate organism (e.g., an organism where theorthogonal components can be used) along with plasmid comprising anO-tRNA of the invention. A cognate synthetase can also be introduced(either as a polypeptide or a polynucleotide that encodes the cognatesynthetase when expressed). The cells are grown in media to a desireddensity, e.g., to an OD₆₀₀ of about 0.5, and β-galactosidase assays areperformed, e.g., using the BetaFluor™ β-Galactosidase Assay Kit(Novagen). Percent suppression can be calculated as the percentage ofactivity for a sample relative to a comparable control, e.g., the valueobserved from the derivatized lacZ construct, where the construct has acorresponding sense codon at desired position rather than a selectorcodon.

Examples of O-tRNAs of the invention are set forth in the sequencelisting herein, for example, see FIG. 2 and SEQ ID NO: 1. The disclosureherein also provides guidance for the design of additional equivalentO-tRNA species. In an RNA molecule, such as an O-RS mRNA, or O-tRNAmolecule, thymine (T) is replace with uracil (U) relative to a givensequence (or vice versa for a coding DNA), or complement thereof.Additional modifications to the bases can also be present to generatelargely functionally equivalent molecules.

The invention also encompasses conservative variations of O-tRNAscorresponding to particular O-tRNAs herein. For example, conservativevariations of O-tRNA include those molecules that function like theparticular O-tRNAs, e.g., as in the sequence listing herein and thatmaintain the tRNA L-shaped structure by virtue of appropriateself-complementarity, but that do not have a sequence identical tothose, e.g., in the sequence listing or FIG. 2, and desirably, are otherthan wild type tRNA molecules.

The composition comprising an O-tRNA can further include an orthogonalaminoacyl-tRNA synthetase (O-RS), where the O-RS preferentiallyaminoacylates the O-tRNA with an unnatural amino acid. In certainembodiments, a composition including an O-tRNA can further include atranslation system (e.g., in vitro or in vivo). A nucleic acid thatcomprises a polynucleotide that encodes a polypeptide of interest, wherethe polynucleotide comprises a selector codon that is recognized by theO-tRNA, or a combination of one or more of these can also be present inthe cell.

Methods of producing an orthogonal tRNA (O-tRNA) are also a feature ofthe invention. An O-tRNA produced by the method is also a feature of theinvention. In certain embodiments of the invention, the O-tRNAs can beproduced by generating a library of mutants. The library of mutant tRNAscan be generated using various mutagenesis techniques known in the art.For example, the mutant tRNAs can be generated by site-specificmutations, random point mutations, homologous recombination, DNAshuffling or other recursive mutagenesis methods, chimeric constructionor any combination thereof, e.g., of the O-tRNA of SEQ ID NO: 1.

Additional mutations can be introduced at a specific position(s), e.g.,at a nonconservative position(s), or at a conservative position, at arandomized position(s), or a combination of both in a desired loop orregion of a tRNA, e.g., an anticodon loop, the acceptor stem, D arm orloop, variable loop, TPC arm or loop, other regions of the tRNAmolecule, or a combination thereof. Typically, mutations in a tRNAinclude mutating the anticodon loop of each member of the library ofmutant tRNAs to allow recognition of a selector codon. The method canfurther include adding additional sequences to the O-tRNA. Typically, anO-tRNA possesses an improvement of orthogonality for a desired organismcompared to the starting material, e.g., the plurality of tRNAsequences, while preserving its affinity towards a desired RS.

The methods optionally include analyzing the similarity (and/or inferredhomology) of sequences of tRNAs and/or aminoacyl-tRNA synthetases todetermine potential candidates for an O-tRNA, O-RS and/or pairs thereof,that appear to be orthogonal for a specific organism. Computer programsknown in the art and described herein can be used for the analysis,e.g., BLAST and pileup programs can be used. In one example, to choosepotential orthogonal translational components for use in E. coli, asynthetase and/or a tRNA is chosen that does not display close sequencesimilarity to eubacterial organisms.

Typically, an O-tRNA is obtained by subjecting to, e.g., negativeselection, a population of cells of a first species, where the cellscomprise a member of the plurality of potential O-tRNAs. The negativeselection eliminates cells that comprise a member of the library ofpotential O-tRNAs that is aminoacylated by an aminoacyl-tRNA synthetase(RS) that is endogenous to the cell. This provides a pool of tRNAs thatare orthogonal to the cell of the first species.

In certain embodiments, in the negative selection, a selector codon(s)is introduced into a polynucleotide that encodes a negative selectionmarker, e.g., an enzyme that confers antibiotic resistance, e.g.,β-lactamase, an enzyme that confers a detectable product, e.g.,β-galactosidase, chloramphenicol acetyltransferase (CAT), e.g., a toxicproduct, such as barnase, at a nonessential position (e.g., stillproducing a functional barnase), etc. Screening/selection is optionallydone by growing the population of cells in the presence of a selectiveagent (e.g., an antibiotic, such as ampicillin). In one embodiment, theconcentration of the selection agent is varied.

For example, to measure the activity of suppressor tRNAs, a selectionsystem is used that is based on the in vivo suppression of selectorcodon, e.g., nonsense (e.g., stop) or frameshift mutations introducedinto a polynucleotide that encodes a negative selection marker, e.g., agene for β-lactamase (bla). For example, polynucleotide variants, e.g.,bla variants, with a selector codon at a certain position (e.g., A184),are constructed. Cells, e.g., bacteria, are transformed with thesepolynucleotides. In the case of an orthogonal tRNA, which cannot beefficiently charged by endogenous E. coli synthetases, antibioticresistance, e.g., ampicillin resistance, should be about or less thanthat for a bacteria transformed with no plasmid. If the tRNA is notorthogonal, or if a heterologous synthetase capable of charging the tRNAis co-expressed in the system, a higher level of antibiotic, e.g.,ampicillin, resistance is be observed. Cells, e.g., bacteria, are chosenthat are unable to grow on LB agar plates with antibiotic concentrationsabout equal to cells transformed with no plasmids.

In the case of a toxic product (e.g., ribonuclease or barnase), when amember of the plurality of potential tRNAs is aminoacylated byendogenous host, e.g., Escherichia coli synthetases (i.e., it is notorthogonal to the host, e.g., Escherichia coli synthetases), theselector codon is suppressed and the toxic polynucleotide productproduced leads to cell death. Cells harboring orthogonal tRNAs ornon-functional tRNAs survive.

In one embodiment, the pool of tRNAs that are orthogonal to a desiredorganism are then subjected to a positive selection in which a selectorcodon is placed in a positive selection marker, e.g., encoded by a drugresistance gene, such a β-lactamase gene. The positive selection isperformed on a cell comprising a polynucleotide encoding or comprising amember of the pool of tRNAs that are orthogonal to the cell, apolynucleotide encoding a positive selection marker, and apolynucleotide encoding a cognate RS. In certain embodiments, the secondpopulation of cells comprises cells that were not eliminated by thenegative selection. The polynucleotides are expressed in the cell andthe cell is grown in the presence of a selection agent, e.g.,ampicillin. tRNAs are then selected for their ability to beaminoacylated by the coexpressed cognate synthetase and to insert anamino acid in response to this selector codon. Typically, these cellsshow an enhancement in suppression efficiency compared to cellsharboring non-functional tRNA(s), or tRNAs that cannot efficiently berecognized by the synthetase of interest. The cell harboring thenon-functional tRNAs or tRNAs that are not efficiently recognized by thesynthetase of interest, are sensitive to the antibiotic. Therefore,tRNAs that: (i) are not substrates for endogenous host, e.g.,Escherichia coli, synthetases; (ii) can be aminoacylated by thesynthetase of interest; and (iii) are functional in translation, surviveboth selections.

Accordingly, the same marker can be either a positive or negativemarker, depending on the context in which it is screened. That is, themarker is a positive marker if it is screened for, but a negative markerif screened against.

The stringency of the selection, e.g., the positive selection, thenegative selection or both the positive and negative selection, in theabove described-methods, optionally includes varying the selectionstringency. For example, because barnase is an extremely toxic protein,the stringency of the negative selection can be controlled byintroducing different numbers of selector codons into the barnase geneand/or by using an inducible promoter. In another example, theconcentration of the selection or screening agent is varied (e.g.,ampicillin concentration). In some aspects of the invention, thestringency is varied because the desired activity can be low duringearly rounds. Thus, less stringent selection criteria are applied inearly rounds and more stringent criteria are applied in later rounds ofselection. In certain embodiments, the negative selection, the positiveselection or both the negative and positive selection can be repeatedmultiple times. Multiple different negative selection markers, positiveselection markers or both negative and positive selection markers can beused. In certain embodiments, the positive and negative selection markercan be the same.

Other types of selections/screening can be used in the invention forproducing orthogonal translational components, e.g., an O-tRNA, an O-RS,and an O-tRNA/O-RS pair that loads an unnatural amino acid in responseto a selector codon. For example, the negative selection marker, thepositive selection marker or both the positive and negative selectionmarkers can include a marker that fluoresces or catalyzes a luminescentreaction in the presence of a suitable reactant. In another embodiment,a product of the marker is detected by fluorescence-activated cellsorting (FACS) or by luminescence. Optionally, the marker includes anaffinity based screening marker. See also, Francisco, J. A., et al.,(1993) Production and fluorescence-activated cell sorting of Escherichiacoli expressing a functional antibody fragment on the external surface.Proc Natl Acad Sci USA. 90:10444-8.

Additional methods for producing a recombinant orthogonal tRNA can befound, e.g., in International Application Publications WO 2002/086075,entitled “METHODS AND COMPOSITIONS FOR THE PRODUCTION OF ORTHOGONAL tRNAAMINOACYL-tRNA SYNTHETASE PAIRS;” WO 2004/094593, entitled “EXPANDINGTHE EUKARYOTIC GENETIC CODE;” and WO 2005/019415, filed Jul. 7, 2004.See also Forster et al., (2003) Programming peptidomimetic synthetasesby translating genetic codes designed de novo PNAS 100(11):6353-6357;and, Feng et al., (2003), Expanding tRNA recognition of a tRNAsynthetase by a single amino acid change, PNAS 100(10): 5676-5681.

Orthogonal Aminoacyl-tRNA Synthetase (O-RS)

An O-RS of the invention preferentially aminoacylates an O-tRNA with anunnatural amino acid, in vitro or in vivo. An O-RS of the invention canbe provided to the translation system, e.g., a cell, by a polypeptidethat includes an O-RS and/or by a polynucleotide that encodes an O-RS ora portion thereof. For example, an example O-RS comprises an amino acidsequence as set forth in SEQ ID NO: 4, 6 or 8, or a conservativevariation thereof. In another example, an O-RS, or a portion thereof, isencoded by a polynucleotide sequence that encodes an amino acidcomprising sequence in the sequence listing or examples herein, or acomplementary polynucleotide sequence thereof. See, e.g., thepolynucleotide of SEQ ID NO: 5, 7 or 9.

Methods for identifying an orthogonal aminoacyl-tRNA synthetase (O-RS),e.g., an O-RS, for use with an O-tRNA, are also a feature of theinvention. For example, a method includes subjecting to selection, e.g.,positive selection, a population of cells of a first species, where thecells individually comprise: 1) a member of a plurality ofaminoacyl-tRNA synthetases (RSs), (e.g., the plurality of RSs caninclude mutant RSs, RSs derived from a species other than the firstspecies or both mutant RSs and RSs derived from a species other than thefirst species); 2) the orthogonal tRNA (O-tRNA) (e.g., from one or morespecies); and 3) a polynucleotide that encodes an (e.g., positive)selection marker and comprises at least one selector codon. Cells areselected or screened for those that show an enhancement in suppressionefficiency compared to cells lacking or with a reduced amount of themember of the plurality of RSs. Suppression efficiency can be measuredby techniques known in the art and as described herein. Cells having anenhancement in suppression efficiency comprise an active RS thataminoacylates the O-tRNA. A level of aminoacylation (in vitro or invivo) by the active RS of a first set of tRNAs from the first species iscompared to the level of aminoacylation (in vitro or in vivo) by theactive RS of a second set of tRNAs from the second species. The level ofaminoacylation can be determined by a detectable substance (e.g., alabeled unnatural amino acid). The active RS that more efficientlyaminoacylates the second set of tRNAs compared to the first set of tRNAsis typically selected, thereby providing an efficient (optimized)orthogonal aminoacyl-tRNA synthetase for use with the O-tRNA. An O-RS,identified by the method, is also a feature of the invention.

Any of a number of assays can be used to determine aminoacylation. Theseassays can be performed in vitro or in vivo. For example, in vitroaminoacylation assays are described in, e.g., Hoben and Soll (1985)Methods Enzymol. 113:55-59. Aminoacylation can also be determined byusing a reporter along with orthogonal translation components anddetecting the reporter in a cell expressing a polynucleotide comprisingat least one selector codon that encodes a protein. See also, WO2002/085923, entitled “IN VIVO INCORPORATION OF UNNATURAL AMINO ACIDS;”and WO 2004/094593, entitiled “EXPANDING THE EUKARYOTIC GENETIC CODE.”

Identified O-RS can be further manipulated to alter substratespecificity of the synthetase, so that only a desired unnatural aminoacid, but not any of the common 20 amino acids, are charged to theO-tRNA. Methods to generate an orthogonal aminoacyl-tRNA synthetase witha substrate specificity for an unnatural amino acid include mutating thesynthetase, e.g., at the active site in the synthetase, at the editingmechanism site in the synthetase, at different sites by combiningdifferent domains of synthetases, or the like, and applying a selectionprocess. A strategy is used, which is based on the combination of apositive selection followed by a negative selection. In the positiveselection, suppression of the selector codon introduced at anonessential position(s) of a positive marker allows cells to surviveunder positive selection pressure. In the presence of both natural andunnatural amino acids, survivors thus encode active synthetases chargingthe orthogonal suppressor tRNA with either a natural or unnatural aminoacid. In the negative selection, suppression of a selector codonintroduced at a nonessential position(s) of a negative marker removessynthetases with natural amino acid specificities. Survivors of thenegative and positive selection encode synthetases that aminoacylate(charge) the orthogonal suppressor tRNA with unnatural amino acids only.These synthetases can then be subjected to further mutagenesis, e.g.,DNA shuffling or other recursive mutagenesis methods.

A library of mutant O-RSs can be generated using various mutagenesistechniques known in the art. For example, the mutant RSs can begenerated by site-specific mutations, random point mutations, homologousrecombination, DNA shuffling or other recursive mutagenesis methods,chimeric construction or any combination thereof. For example, a libraryof mutant RSs can be produced from two or more other, e.g., smaller,less diverse “sub-libraries.” Chimeric libraries of RSs are alsoincluded in the invention. It should be noted that libraries of tRNAsynthetases from various organism (e.g., microorganisms such aseubacteria or archaebacteria) such as libraries that comprise naturaldiversity (see, e.g., U.S. Pat. No. 6,238,884 to Short et al; U.S. Pat.No. 5,756,316 to Schallenberger et al; U.S. Pat. No. 5,783,431 toPetersen et al; U.S. Pat. No. 5,824,485 to Thompson et al; U.S. Pat. No.5,958,672 to Short et al), are optionally constructed and screened fororthogonal pairs.

Once the synthetases are subject to the positive and negativeselection/screening strategy, these synthetases can then be subjected tofurther mutagenesis. For example, a nucleic acid that encodes the O-RScan be isolated; a set of polynucleotides that encode mutated O-RSs(e.g., by random mutagenesis, site-specific mutagenesis, recombinationor any combination thereof) can be generated from the nucleic acid; and,these individual steps or a combination of these steps can be repeateduntil a mutated O-RS is obtained that preferentially aminoacylates theO-tRNA with the unnatural amino acid. In some aspects of the invention,the steps are performed multiple times, e.g., at least two times.

Additional levels of selection/screening stringency can also be used inthe methods of the invention, for producing O-tRNA, O-RS, or pairsthereof. The selection or screening stringency can be varied on one orboth steps of the method to produce an O-RS. This could include, e.g.,varying the amount of selection/screening agent that is used, etc.Additional rounds of positive and/or negative selections can also beperformed. Selecting or screening can also comprise one or more of achange in amino acid permeability, a change in translation efficiency, achange in translational fidelity, etc. Typically, the one or more changeis based upon a mutation in one or more gene in an organism in which anorthogonal tRNA-tRNA synthetase pair is used to produce protein.

Additional general details for producing O-RS, and altering thesubstrate specificity of the synthetase can be found in InternalPublication Number WO 2002/086075, entitled “METHODS AND COMPOSITIONSFOR THE PRODUCTION OF ORTHOGONAL tRNA AMINOACYL-tRNA SYNTHETASE PAIRS;”and WO 2004/094593, entitled “EXPANDING THE EUKARYOTIC GENETIC CODE.”See also, Wang and Schultz “Expanding the Genetic Code,” AngewandteChemie Int. Ed., 44(1):34-66 (2005), the content of which isincorporated by reference in its entirety.

Source and Host Organisms

The orthogonal translational components (O-tRNA and O-RS) of theinvention can be derived from any organism (or a combination oforganisms) for use in a host translation system from any other species,with the caveat that the O-tRNA/O-RS components and the host system workin an orthogonal manner. It is not a requirement that the O-tRNA and theO-RS from an orthogonal pair be derived from the same organism. In someaspects, the orthogonal components are derived from Archaea genes (i.e.,archaebacteria) for use in a eubacterial host system.

For example, the orthogonal O-tRNA can be derived from an Archaeorganism, e.g., an archaebacterium, such as Methanococcus jannaschii,Methanobacterium thermoautotrophicum, Halobacterium such as Haloferaxvolcanii and Halobacterium species NRC-1, Archaeoglobus fulgidus,Pyrococcus furiosus, Pyrococcus horikoshii, Aeuropyrum pernix,Methanococcus maripaludis, Methanopyrus kandleri, Methanosarcina mazei(Mm), Pyrobaculum aerophilum, Pyrococcus abyssi, Sulfolobus solfataricus(Ss), Sulfolobus tokodaii, Thermoplasma acidophilum, Thermoplasmavolcanium, or the like, or a eubacterium, such as Escherichia coli,Thermus thermophilus, Bacillus subtilis, Bacillus stearothermphilus, orthe like, while the orthogonal O-RS can be derived from an organism orcombination of organisms, e.g., an archaebacterium, such asMethanococcus jannaschii, Methanobacterium thermoautotrophicum,Halobacterium such as Haloferax volcanii and Halobacterium speciesNRC-1, Archaeoglobus fulgidus, Pyrococcus furiosus, Pyrococcushorikoshii, Aeuropyrum pernix, Methanococcus maripaludis, Methanopyruskandleri, Methanosarcina mazei, Pyrobaculum aerophilum, Pyrococcusabyssi, Sulfolobus solfataricus, Sulfolobus tokodaii, Thermoplasmaacidophilum, Thermoplasma volcanium, or the like, or a eubacterium, suchas Escherichia coli, Thermus thermophilus, Bacillus subtilis, Bacillusstearothermphilus, or the like. In one embodiment, eukaryotic sources,e.g., plants, algae, protists, fungi, yeasts, animals (e.g., mammals,insects, arthropods, etc.), or the like, can also be used as sources ofO-tRNAs and O-RSs.

The individual components of an O-tRNA/O-RS pair can be derived from thesame organism or different organisms. In one embodiment, the O-tRNA/O-RSpair is from the same organism. Alternatively, the O-tRNA and the O-RSof the O-tRNA/O-RS pair are from different organisms.

The O-tRNA, O-RS or O-tRNA/O-RS pair can be selected or screened in vivoor in vitro and/or used in a cell, e.g., a eubacterial cell, to producea polypeptide with an unnatural amino acid. The eubacterial cell used isnot limited, for example, Escherichia coli, Thermus thermophilus,Bacillus subtilis, Bacillus stearothermphilus, or the like. Compositionsof eubacterial cells comprising translational components of theinvention are also a feature of the invention.

See also, International Application Publication Number WO 2004/094593,entitled “EXPANDING THE EUKARYOTIC GENETIC CODE,” filed Apr. 16, 2004,for screening O-tRNA and/or O-RS in one species for use in anotherspecies.

Although orthogonal translation systems (e.g., comprising an O-RS, anO-tRNA and an unnatural amino acid) can utilize cultured host cells toproduce proteins having unnatural amino acids, it is not intended thatan orthogonal translation system of the invention require an intact,viable host cell. For example, a orthogonal translation system canutilize a cell-free system in the presence of a cell extract. Indeed,the use of cell free, in vitro transcription/translation systems forprotein production is a well established technique. Adaptation of thesein vitro systems to produce proteins having unnatural amino acids usingorthogonal translation system components described herein is well withinthe scope of the invention.

Selector Codons

Selector codons of the invention expand the genetic codon framework ofprotein biosynthetic machinery. For example, a selector codon includes,e.g., a unique three base codon, a nonsense codon, such as a stop codon,e.g., an amber codon (UAG), or an opal codon (UGA), an unnatural codon,at least a four base codon, a rare codon, or the like. A number ofselector codons can be introduced into a desired gene, e.g., one ormore, two or more, more than three, etc. By using different selectorcodons, multiple orthogonal tRNA/synthetase pairs can be used that allowthe simultaneous site-specific incorporation of multiple unnatural aminoacids e.g., including at least one unnatural amino acid, using thesedifferent selector codons.

In one embodiment, the methods involve the use of a selector codon thatis a stop codon for the incorporation of an unnatural amino acid in vivoin a cell. For example, an O-tRNA is produced that recognizes the stopcodon and is aminoacylated by an O-RS with an unnatural amino acid. ThisO-tRNA is not recognized by the naturally occurring host'saminoacyl-tRNA synthetases. Conventional site-directed mutagenesis canbe used to introduce the stop codon at the site of interest in apolynucleotide encoding a polypeptide of interest. See, e.g., Sayers, J.R., et al. (1988), 5′,3′ Exonuclease in phosphorothioate-basedoligonucleotide-directed mutagenesis. Nucleic Acids Res, 791-802. Whenthe O-RS, O-tRNA and the nucleic acid that encodes a polypeptide ofinterest are combined, e.g., in vivo, the unnatural amino acid isincorporated in response to the stop codon to give a polypeptidecontaining the unnatural amino acid at the specified position. In oneembodiment of the invention, the stop codon used as a selector codon isan amber codon, UAG, and/or an opal codon, UGA. In one example, agenetic code in which UAG and UGA are both used as a selector codon canencode 22 amino acids while preserving the ochre nonsense codon, UAA,which is the most abundant termination signal.

The incorporation of unnatural amino acids in vivo can be done withoutsignificant perturbation of the host cell. For example in non-eukaryoticcells, such as Escherichia coli, because the suppression efficiency forthe UAG codon depends upon the competition between the O-tRNA, e.g., theamber suppressor tRNA, and the release factor 1 (RF1) (which binds tothe UAG codon and initiates release of the growing peptide from theribosome), the suppression efficiency can be modulated by, e.g., eitherincreasing the expression level of O-tRNA, e.g., the suppressor tRNA, orusing an RF1 deficient strain. In eukaryotic cells, because thesuppression efficiency for the UAG codon depends upon the competitionbetween the O-tRNA, e.g., the amber suppressor tRNA, and a eukaryoticrelease factor (e.g., eRF) (which binds to a stop codon and initiatesrelease of the growing peptide from the ribosome), the suppressionefficiency can be modulated by, e.g., increasing the expression level ofO-tRNA, e.g., the suppressor tRNA. In addition, additional compounds canalso be present, e.g., reducing agents such as dithiothretiol (DTT).

Unnatural amino acids can also be encoded with rare codons. For example,when the arginine concentration in an in vitro protein synthesisreaction is reduced, the rare arginine codon, AGG, has proven to beefficient for insertion of Ala by a synthetic tRNA acylated withalanine. See, e.g., Ma et al., Biochemistry, 32:7939 (1993). In thiscase, the synthetic tRNA competes with the naturally occurringtRNA^(Arg), which exists as a minor species in Escherichia coli. Inaddition, some organisms do not use all triplet codons. An unassignedcodon AGA in Micrococcus luteus has been utilized for insertion of aminoacids in an in vitro transcription/translation extract. See, e.g., Kowaland Oliver, Nucl. Acid. Res., 25:4685 (1997). Components of theinvention can be generated to use these rare codons in vivo.

Selector codons can also comprise extended codons, e.g., four or morebase codons, such as, four, five, six or more base codons. Examples offour base codons include, e.g., AGGA, CUAG, UAGA, CCCU, and the like.Examples of five base codons include, e.g., AGGAC, CCCCU, CCCUC, CUAGA,CUACU, UAGGC and the like. Methods of the invention include usingextended codons based on frameshift suppression. Four or more basecodons can insert, e.g., one or multiple unnatural amino acids, into thesame protein. In other embodiments, the anticodon loops can decode,e.g., at least a four-base codon, at least a five-base codon, or atleast a six-base codon or more. Since there are 256 possible four-basecodons, multiple unnatural amino acids can be encoded in the same cellusing a four or more base codon. See also, Anderson et al., (2002)Exploring the Limits of Codon and Anticodon Size, Chemistry and Biology,9:237-244; and, Magliery, (2001) Expanding the Genetic Code: Selectionof Efficient Suppressors of Four-base Codons and Identification of“Shifty” Four-base Codons with a Library Approach in Escherichia coli,J. Mol. Biol. 307: 755-769.

For example, four-base codons have been used to incorporate unnaturalamino acids into proteins using in vitro biosynthetic methods. See,e.g., Ma et al., (1993) Biochemistry, 32:7939; and Hohsaka et al.,(1999) J. Am. Chem. Soc., 121:34. CGGG and AGGU were used tosimultaneously incorporate 2-naphthylalanine and an NBD derivative oflysine into streptavidin in vitro with two chemically acylatedframeshift suppressor tRNAs. See, e.g., Hohsaka et al., (1999) J. Am.Chem. Soc., 121:12194. In an in vivo study, Moore et al. examined theability of tRNA^(Leu) derivatives with NCUA anticodons to suppress UAGNcodons (N can be U, A, G, or C), and found that the quadruplet UAGA canbe decoded by a tRNA^(Leu) with a UCUA anticodon with an efficiency of13 to 26% with little decoding in the 0 or −1 frame. See Moore et al.,(2000) J. Mol. Biol., 298:195. In one embodiment, extended codons basedon rare codons or nonsense codons can be used in invention, which canreduce missense readthrough and frameshift suppression at other unwantedsites. Four base codons have been used as selector codons in a varietyof orthogonal systems. See, e.g., WO 2005/019415; WO 2005/007870 and WO2005/07624. See also, Wang and Schultz “Expanding the Genetic Code,”Angewandte Chemie Int. Ed., 44(1):34-66 (2005), the content of which isincorporated by reference in its entirety. While the examples belowutilize an amber selector codon, four or more base codons can be used aswell, by modifying the examples herein to include four-base O-tRNAs andsynthetases modified to include mutations similar to those previouslydescribed for various unnatural amino acid O-RSs.

For a given system, a selector codon can also include one of the naturalthree base codons, where the endogenous system does not use (or rarelyuses) the natural base codon. For example, this includes a system thatis lacking a tRNA that recognizes the natural three base codon, and/or asystem where the three base codon is a rare codon.

Selector codons optionally include unnatural base pairs. These unnaturalbase pairs further expand the existing genetic alphabet. One extra basepair increases the number of triplet codons from 64 to 125. Propertiesof third base pairs include stable and selective base pairing, efficientenzymatic incorporation into DNA with high fidelity by a polymerase, andthe efficient continued primer extension after synthesis of the nascentunnatural base pair. Descriptions of unnatural base pairs which can beadapted for methods and compositions include, e.g., Hirao, et al.,(2002) An unnatural base pair for incorporating amino acid analoguesinto protein, Nature Biotechnology, 20:177-182. See also Wu, Y., et al.,(2002) J. Am. Chem. Soc. 124:14626-14630. Other relevant publicationsare listed below.

For in vivo usage, the unnatural nucleoside is membrane permeable and isphosphorylated to form the corresponding triphosphate. In addition, theincreased genetic information is stable and not destroyed by cellularenzymes. Previous efforts by Benner and others took advantage ofhydrogen bonding patterns that are different from those in canonicalWatson-Crick pairs, the most noteworthy example of which is theiso-C:iso-G pair. See, e.g., Switzer et al., (1989) J. Am. Chem. Soc.,111:8322; and Piccirilli et al., (1990) Nature, 343:33; Kool, (2000)Curr. Opin. Chem. Biol., 4:602. These bases in general mispair to somedegree with natural bases and cannot be enzymatically replicated. Kooland co-workers demonstrated that hydrophobic packing interactionsbetween bases can replace hydrogen bonding to drive the formation ofbase pair. See Kool, (2000) Curr. Opin. Chem. Biol., 4:602; and Guckianand Kool, (1998) Angew. Chem. Int. Ed. Engl., 36, 2825. In an effort todevelop an unnatural base pair satisfying all the above requirements,Schultz, Romesberg and co-workers have systematically synthesized andstudied a series of unnatural hydrophobic bases. A PICS:PICS self-pairis found to be more stable than natural base pairs, and can beefficiently incorporated into DNA by Klenow fragment of Escherichia coliDNA polymerase I (KF). See, e.g., McMinn et al., (1999) J. Am. Chem.Soc., 121:11586; and Ogawa et al., (2000) J. Am. Chem. Soc., 122:3274. A3MN:3MN self-pair can be synthesized by KF with efficiency andselectivity sufficient for biological function. See, e.g., Ogawa et al.,(2000) J. Am. Chem. Soc., 122:8803. However, both bases act as a chainterminator for further replication. A mutant DNA polymerase has beenrecently evolved that can be used to replicate the PICS self pair. Inaddition, a 7AI self pair can be replicated. See, e.g., Tae et al.,(2001) J. Am. Chem. Soc., 123:7439. A novel metallobase pair, Dipic:Py,has also been developed, which forms a stable pair upon binding Cu(II).See Meggers et al., (2000) J. Am. Chem. Soc., 122:10714. Becauseextended codons and unnatural codons are intrinsically orthogonal tonatural codons, the methods of the invention can take advantage of thisproperty to generate orthogonal tRNAs for them.

A translational bypassing system can also be used to incorporate anunnatural amino acid in a desired polypeptide. In a translationalbypassing system, a large sequence is inserted into a gene but is nottranslated into protein. The sequence contains a structure that servesas a cue to induce the ribosome to hop over the sequence and resumetranslation downstream of the insertion.

Unnatural Amino Acids

As used herein, an unnatural amino acid refers to any amino acid,modified amino acid, or amino acid analogue other than selenocysteineand/or pyrrolysine and the following twenty genetically encodedalpha-amino acids: alanine, arginine, asparagine, aspartic acid,cysteine, glutamine, glutamic acid, glycine, histidine, isoleucine,leucine, lysine, methionine, phenylalanine, proline, serine, threonine,tryptophan, tyrosine, valine. The generic structure of an alpha-aminoacid is illustrated by Formula I:

An unnatural amino acid is typically any structure having Formula Iwherein the R group is any substituent other than one used in the twentynatural amino acids. See e.g., Biochemistry by L. Stryer, 3^(rd) ed.1988, Freeman and Company, New York, for structures of the twentynatural amino acids. Note that, the unnatural amino acids of theinvention can be naturally occurring compounds other than the twentyalpha-amino acids above.

Because the unnatural amino acids of the invention typically differ fromthe natural amino acids in side chain, the unnatural amino acids formamide bonds with other amino acids, e.g., natural or unnatural, in thesame manner in which they are formed in naturally occurring proteins.However, the unnatural amino acids have side chain groups thatdistinguish them from the natural amino acids.

Of particular interest herein is the unnatural amino acidphenylselenocysteine (see FIG. 1, structure 1). In addition to thephenylselenocysteine unnatural amino acid, other unnatural amino acidscan be simultaneously incorporated into a polypeptide of interest, e.g.,using an appropriate second O-RS/O-tRNA pair in conjunction with anorthogonal pair provided by the present invention. Many such additionalunnatural amino acids and suitable orthogonal pairs are known. See thepresent disclosure and the references cited herein. For example, seeWang and Schultz “Expanding the Genetic Code,” Angewandte Chemie Int.Ed., 44(1):34-66 (2005); Xie and Schultz, “An Expanding Genetic Code,”Methods 36(3):227-238 (2005); Xie and Schultz, “Adding Amino Acids tothe Genetic Repertoire,” Curr. Opinion in Chemical Biology 9(6):548-554(2005); and Wang et al., “Expanding the Genetic Code,” Annu. Rev.Biophys. Biomol. Struct., 35:225-249 (2006); the contents of which areeach incorporated by reference in their entirety.

Although the phenylselenocysteine unnatural amino acid shown in FIG. 1,structure 1, is of primary interest in the Examples described herein, itis not intended that the invention be strictly limited to thatstructure. Indeed, a variety of easily-derived, structurally relatedanalogs can be readily produced that retain the principle characteristicof the phenylselenocysteine shown in FIG. 1, structure 1, and also arespecifically recognized by the aminoacyl-tRNA synthetases of theinvention (e.g., the O-RS of SEQ ID NOS: 4, 6 and 8). It is intendedthat these related amino acid analogues are within the scope of theinvention.

In other unnatural amino acids, for example, R in Formula I optionallycomprises an alkyl-, aryl-, acyl-, hydrazine, cyano-, halo-, hydrazide,alkenyl, ether, borate, boronate, phospho, phosphono, phosphine, enone,imine, ester, hydroxylamine, amine, and the like, or any combinationthereof. Other unnatural amino acids of interest include, but are notlimited to, amino acids comprising a photoactivatable cross-linker,spin-labeled amino acids, fluorescent amino acids, metal binding aminoacids, metal-containing amino acids, radioactive amino acids, aminoacids with novel functional groups, amino acids that covalently ornoncovalently interact with other molecules, photocaged and/orphotoisomerizable amino acids, biotin or biotin-analogue containingamino acids, keto containing amino acids, glycosylated amino acids, asaccharide moiety attached to the amino acid side chain, amino acidscomprising polyethylene glycol or polyether, heavy atom substitutedamino acids, chemically cleavable or photocleavable amino acids, aminoacids with an elongated side chain as compared to natural amino acids(e.g., polyethers or long chain hydrocarbons, e.g., greater than about5, greater than about 10 carbons, etc.), carbon-linked sugar-containingamino acids, amino thioacid containing amino acids, and amino acidscontaining one or more toxic moiety.

In another aspect, the invention provides unnatural amino acids havingthe general structure illustrated by Formula IV below:

An unnatural amino acid having this structure is typically any structurewhere R₁ is a substituent used in one of the twenty natural amino acids(e.g., tyrosine or phenylalanine) and R₂ is a substituent. Thus, thistype of unnatural amino acid can be viewed as a natural amino acidderivative.

In addition to unnatural amino acids that contain thephenylselenocysteine structure shown in FIG. 1, structure 1, unnaturalamino acids can also optionally comprise modified backbone structures,e.g., as illustrated by the structures of Formula II and III:

wherein Z typically comprises OH, NH₂, SH, NH—R′, or S—R′; X and Y,which can be the same or different, typically comprise S or O, and R andR′, which are optionally the same or different, are typically selectedfrom the same list of constituents for the R group described above forthe unnatural amino acids having Formula I as well as hydrogen. Forexample, unnatural amino acids of the invention optionally comprisesubstitutions in the amino or carboxyl group as illustrated by FormulasII and III. Unnatural amino acids of this type include, but are notlimited to, α-hydroxy acids, α-thioacids α-aminothiocarboxylates, e.g.,with side chains corresponding to the common twenty natural amino acidsor unnatural side chains. In addition, substitutions at the α-carbonoptionally include L, D, or α-α-disubstituted amino acids such asD-glutamate, D-alanine, D-methyl-O-tyrosine, aminobutyric acid, and thelike. Other structural alternatives include cyclic amino acids, such asproline analogues as well as 3, 4, 6, 7, 8, and 9 membered ring prolineanalogues, β and γ amino acids such as substituted β-alanine and γ-aminobutyric acid.

In some aspects, the invention utilizes unnatural amino acids in theL-configuration. However, it is not intended that the invention belimited to the use of L-configuration unnatural amino acids. It iscontemplated that the D-enantiomers of these unnatural amino acids alsofind use with the invention.

The unnatural amino acids finding use with the invention is not strictlylimited to the phenylselenocysteine unnatural amino acid shown in FIG.1, structure 1. One of skill in the art will recognize that a widevariety of unnatural analogs of naturally occurring amino acids areeasily derived. For example, but not limited to, unnatural derived fromtyrosine are readily produced. Tyrosine analogs include, e.g.,para-substituted tyrosines, ortho-substituted tyrosines, and metasubstituted tyrosines, wherein the substituted tyrosine comprises analkynyl group, acetyl group, a benzoyl group, an amino group, ahydrazine, an hydroxyamine, a thiol group, a carboxy group, an isopropylgroup, a methyl group, a C₆-C₂₀ straight chain or branched hydrocarbon,a saturated or unsaturated hydrocarbon, an O-methyl group, a polyethergroup, a nitro group, or the like. In addition, multiply substitutedaryl rings are also contemplated. Glutamine analogs of the inventioninclude, but are not limited to, α-hydroxy derivatives, γ-substitutedderivatives, cyclic derivatives, and amide substituted glutaminederivatives. Example phenylalanine analogs include, but are not limitedto, para-substituted phenylalanines, ortho-substituted phenyalanines,and meta-substituted phenylalanines, wherein the substituent comprisesan alkynyl group, a hydroxy group, a methoxy group, a methyl group, anallyl group, an aldehyde, a nitro, a thiol group, or keto group, or thelike. Specific examples of unnatural amino acids include, but are notlimited to, phenylselenocysteine, sulfotyrosine,p-ethylthiocarbonyl-L-phenylalanine, p-(3-oxobutanoyl)-L-phenylalanine,1,5-dansyl-alanine, 7-amino-coumarin amino acid, 7-hydroxy-coumarinamino acid, nitrobenzyl-serine, O-(2-nitrobenzyl)-L-tyrosine,p-carboxymethyl-L-phenylalanine, p-cyano-L-phenylalanine,m-cyano-L-phenylalanine, biphenylalanine, 3-amino-L-tyrosine, bipyridylalanine, p-(2-amino-1-hydroxyethyl)-L-phenylalanine,p-isopropylthiocarbonyl-L-phenylalanine, 3-nitro-L-tyrosine andp-nitro-L-phenylalanine. Also, a p-propargyloxyphenylalanine, a3,4-dihydroxy-L-phenyalanine (DHP), a 3,4,6-trihydroxy-L-phenylalanine,a 3,4,5-trihydroxy-L-phenylalanine, 4-nitro-phenylalanine, ap-acetyl-L-phenylalanine, O-methyl-L-tyrosine, anL-3-(2-naphthyl)alanine, a 3-methyl-phenylalanine, anO-4-allyl-L-tyrosine, a 4-propyl-L-tyrosine, a 3-nitro-tyrosine, a3-thiol-tyrosine, a tri-O-acetyl-GlcNAcβ-serine, an L-Dopa, afluorinated phenylalanine, an isopropyl-L-phenylalanine, ap-azido-L-phenylalanine, a p-acyl-L-phenylalanine, ap-benzoyl-L-phenylalanine, an L-phosphoserine, a phosphonoserine, aphosphonotyrosine, a p-iodo-phenylalanine, a p-bromophenylalanine, ap-amino-L-phenylalanine, and an isopropyl-L-phenylalanine, and the like.The structures of a variety of unnatural amino acids are disclosed inthe references cited herein. See also, Published InternationalApplications WO 2004/094593, entitled “EXPANDING THE EUKARYOTIC GENETICCODE;” and WO 2006/110182, entitled “ORTHOGONAL TRANSLATION COMPONENTSFOR THE IN VIVO INCORPORATION OF UNNATURAL AMINO ACIDS,” filed Oct. 27,2005.

Chemical Synthesis of Unnatural Amino Acids

Many of the unnatural amino acids provided above are commerciallyavailable, e.g., from Sigma (USA) or Aldrich (Milwaukee, Wis., USA).Those that are not commercially available are optionally synthesized asprovided in various publications or using standard methods known tothose of skill in the art. For organic synthesis techniques, see, e.g.,Organic Chemistry by Fessendon and Fessendon, (1982, Second Edition,Willard Grant Press, Boston Mass.); Advanced Organic Chemistry by March(Third Edition, 1985, Wiley and Sons, New York); and Advanced OrganicChemistry by Carey and Sundberg (Third Edition, Parts A and B, 1990,Plenum Press, New York). Additional publications describing thesynthesis of unnatural amino acids include, e.g., WO 2002/085923entitled “In vivo incorporation of Unnatural Amino Acids;” Matsoukas etal., (1995) J. Med. Chem., 38, 4660-4669; King and Kidd (1949) A NewSynthesis of Glutamine and of γ-Dipeptides of Glutamic Acid fromPhthylated Intermediates. J. Chem. Soc., 3315-3319; Friedman andChatterrji (1959) Synthesis of Derivatives of Glutamine as ModelSubstratesfor Anti-Tumor Agents. J. Am. Chem. Soc. 81, 3750-3752; Craiget al. (1988) Absolute Configuration of the Enantiomers of7-Chloro-4[[4-(diethylamino)-1-methylbutyl]amino]quinoline(Chloroquine). J. Org. Chem. 53, 1167-1170; Azoulay et al. (1991)Glutamine analogues as Potential Antimalarials, Eur. J. Med. Chem. 26,201-5; Koskinen, and Rapoport (1989) Synthesis of 4-Substituted Prolinesas Conformationally Constrained Amino Acid Analogues. J. Org. Chem. 54,1859-1866; Christie and Rapoport (1985) Synthesis of Optically PurePipecolates from L-Asparagine. Application to the Total Synthesis of(+)-Apovincamine through Amino Acid Decarbonylation and Iminium IonCyclization. J. Org. Chem. 1989:1859-1866; Barton et al., (1987)Synthesis of Novel a-Amino-Acids and Derivatives Using RadicalChemistry: Synthesis of L- and D-a-Amino-Adipic Acids, L-a-aminopimelicAcid and Appropriate Unsaturated Derivatives. Tetrahedron Lett.43:4297-4308; and, Subasinghe et al., (1992) Quisqualic acid analogues:synthesis of beta-heterocyclic 2-aminopropanoic acid derivatives andtheir activity at a novel quisqualate-sensitized site. J. Med. Chem.35:4602-7. See also, International Publication WO 2004/058946, entitled“PROTEIN ARRAYS,” filed on Dec. 22, 2003.

Cellular Uptake of Unnatural Amino Acids

Unnatural amino acid uptake by a cell is one issue that is typicallyconsidered when designing and selecting unnatural amino acids, e.g., forincorporation into a protein. For example, the high charge density ofα-amino acids suggests that these compounds are unlikely to be cellpermeable. Natural amino acids are taken up into the cell via acollection of protein-based transport systems often displaying varyingdegrees of amino acid specificity. A rapid screen can be done whichassesses which unnatural amino acids, if any, are taken up by cells.See, e.g., the toxicity assays in, e.g., International Publication WO2004/058946, entitled “PROTEIN ARRAYS,” filed on Dec. 22, 2003; and Liuand Schultz (1999) Progress toward the evolution of an organism with anexpanded genetic code. PNAS 96:4780-4785. Although uptake is easilyanalyzed with various assays, an alternative to designing unnaturalamino acids that are amenable to cellular uptake pathways is to providebiosynthetic pathways to create amino acids in vivo.

Biosynthesis of Unnatural Amino Acids

Many biosynthetic pathways already exist in cells for the production ofamino acids and other compounds. While a biosynthetic method for aparticular unnatural amino acid may not exist in nature, e.g., in acell, the invention provides such methods. For example, biosyntheticpathways for unnatural amino acids are optionally generated in host cellby adding new enzymes or modifying existing host cell pathways.Additional new enzymes are optionally naturally occurring enzymes orartificially evolved enzymes. For example, the biosynthesis ofp-aminophenylalanine (as presented in an example in WO 2002/085923)relies on the addition of a combination of known enzymes from otherorganisms. The genes for these enzymes can be introduced into a cell bytransforming the cell with a plasmid comprising the genes. The genes,when expressed in the cell, provide an enzymatic pathway to synthesizethe desired compound. Examples of the types of enzymes that areoptionally added are provided in the examples below. Additional enzymessequences are found, e.g., in Genbank. Artificially evolved enzymes arealso optionally added into a cell in the same manner. In this manner,the cellular machinery and resources of a cell are manipulated toproduce unnatural amino acids.

Indeed, any of a variety of methods can be used for producing novelenzymes for use in biosynthetic pathways, or for evolution of existingpathways, for the production of unnatural amino acids, in vitro or invivo. Many available methods of evolving enzymes and other biosyntheticpathway components can be applied to the present invention to produceunnatural amino acids (or, indeed, to evolve synthetases to have newsubstrate specificities or other activities of interest). For example,DNA shuffling is optionally used to develop novel enzymes and/orpathways of such enzymes for the production of unnatural amino acids (orproduction of new synthetases), in vitro or in vivo. See, e.g., Stemmer(1994), Rapid evolution of a protein in vitro by DNA shuffling, Nature370(4):389-391; and, Stemmer, (1994), DNA shuffling by randomfragmentation and reassembly: In vitro recombination for molecularevolution, Proc. Natl. Acad. Sci. USA., 91:10747-10751. A relatedapproach shuffles families of related (e.g., homologous) genes toquickly evolve enzymes with desired characteristics. An example of such“family gene shuffling” methods is found in Crameri et al. (1998) “DNAshuffling of a family of genes from diverse species accelerates directedevolution” Nature, 391(6664): 288-291. New enzymes (whether biosyntheticpathway components or synthetases) can also be generated using a DNArecombination procedure known as “incremental truncation for thecreation of hybrid enzymes” (“ITCHY”), e.g., as described in Ostermeieret al. (1999) “A combinatorial approach to hybrid enzymes independent ofDNA homology” Nature Biotech 17:1205. This approach can also be used togenerate a library of enzyme or other pathway variants which can serveas substrates for one or more in vitro or in vivo recombination methods.See, also, Ostermeier et al. (1999) “Combinatorial Protein Engineeringby Incremental Truncation,” Proc. Natl. Acad. Sci. USA, 96: 3562-67, andOstermeier et al. (1999), “Incremental Truncation as a Strategy in theEngineering of Novel Biocatalysts,” Biological and Medicinal Chemistry,7: 2139-44. Another approach uses exponential ensemble mutagenesis toproduce libraries of enzyme or other pathway variants that are, e.g.,selected for an ability to catalyze a biosynthetic reaction relevant toproducing an unnatural amino acid (or a new synthetase). In thisapproach, small groups of residues in a sequence of interest arerandomized in parallel to identify, at each altered position, aminoacids which lead to functional proteins. Examples of such procedures,which can be adapted to the present invention to produce new enzymes forthe production of unnatural amino acids (or new synthetases) are foundin Delegrave and Youvan (1993) Biotechnology Research 11: 1548-1552. Inyet another approach, random or semi-random mutagenesis using doped ordegenerate oligonucleotides for enzyme and/or pathway componentengineering can be used, e.g., by using the general mutagenesis methodsof e.g., Arkin and Youvan (1992) “Optimizing nucleotide mixtures toencode specific subsets of amino acids for semi-random mutagenesis”Biotechnology 10:297-300; or Reidhaar-Olson et al. (1991) “Randommutagenesis of protein sequences using oligonucleotide cassettes”Methods Enzymol. 208:564-86. Yet another approach, often termed a“non-stochastic” mutagenesis, which uses polynucleotide reassembly andsite-saturation mutagenesis can be used to produce enzymes and/orpathway components, which can then be screened for an ability to performone or more synthetase or biosynthetic pathway function (e.g., for theproduction of unnatural amino acids in vivo). See, e.g., Short“NON-STOCHASTIC GENERATION OF GENETIC VACCINES AND ENZYMES” WO 00/46344.

An alternative to such mutational methods involves recombining entiregenomes of organisms and selecting resulting progeny for particularpathway functions (often referred to as “whole genome shuffling”). Thisapproach can be applied to the present invention, e.g., by genomicrecombination and selection of an organism (e.g., an E. coli or othercell) for an ability to produce an unnatural amino acid (or intermediatethereof). For example, methods taught in the following publications canbe applied to pathway design for the evolution of existing and/or newpathways in cells to produce unnatural amino acids in vivo: Patnaik etal. (2002) “Genome shuffling of lactobacillus for improved acidtolerance” Nature Biotechnology 20(7):707-712; and Zhang et al. (2002)“Genome Shuffling Leads to Rapid Phenotypic Improvement in Bacteria”Nature 415(6872):644-646.

Other techniques for organism and metabolic pathway engineering, e.g.,for the production of desired compounds are also available and can alsobe applied to the production of unnatural amino acids. Examples ofpublications teaching useful pathway engineering approaches include:Nakamura and White (2003) “Metabolic engineering for the microbialproduction of 1,3 propanediol” Curr. Opin. Biotechnol. 14(5):454-9;Berry et al. (2002) “Application of Metabolic Engineering to improveboth the production and use of Biotech Indigo” J. IndustrialMicrobiology and Biotechnology 28:127-133; Banta et al. (2002)“Optimizing an artificial metabolic pathway: Engineering the cofactorspecificity of Corynebacterium 2,5-diketo-D-gluconic acid reductase foruse in vitamin C biosynthesis” Biochemistry, 41(20), 6226-36; Selivonovaet al. (2001) “Rapid Evolution of Novel Traits in Microorganisms”Applied and Environmental Microbiology, 67:3645, and many others.

Regardless of the method used, typically, the unnatural amino acidproduced with an engineered biosynthetic pathway of the invention isproduced in a concentration sufficient for efficient proteinbiosynthesis, e.g., a natural cellular amount, but not to such a degreeas to significantly affect the concentration of other cellular aminoacids or to exhaust cellular resources. Typical concentrations producedin vivo in this manner are about 10 mM to about 0.05 mM. Once a cell isengineered to produce enzymes desired for a specific pathway and anunnatural amino acid is generated, in vivo selections are optionallyused to further optimize the production of the unnatural amino acid forboth ribosomal protein synthesis and cell growth.

Orthogonal Components for Incorporating Unnatural Amino Acids

The invention provides compositions and methods for producing orthogonalcomponents for incorporating the unnatural amino acidphenylselenocysteine (see FIG. 1, structure 1) into a growingpolypeptide chain in response to a selector codon, e.g., an amber stopcodon, a nonsense codon, a four or more base codon, etc., e.g., in vivo.For example, the invention provides orthogonal-tRNAs (O-tRNAs),orthogonal aminoacyl-tRNA synthetases (O-RSs) and pairs thereof. Thesepairs can be used to incorporate an unnatural amino acid into growingpolypeptide chains.

A composition of the invention includes an orthogonal aminoacyl-tRNAsynthetase (O-RS), where the O-RS preferentially aminoacylates an O-tRNAwith phenylselenocysteine. In certain embodiments, the O-RS comprises anamino acid sequence comprising SEQ ID NO: 4, 6 or 8, and conservativevariations thereof. In certain embodiments of the invention, the O-RSpreferentially aminoacylates the O-tRNA over any endogenous tRNA with anthe particular unnatural amino acid, where the O-RS has a bias for theO-tRNA, and where the ratio of O-tRNA charged with an unnatural aminoacid to the endogenous tRNA charged with the same unnatural amino acidis greater than 1:1, and more preferably where the O-RS charges theO-tRNA exclusively or nearly exclusively.

A composition that includes an O-RS can optionally further include anorthogonal tRNA (O-tRNA), where the O-tRNA recognizes a selector codon.Typically, an O-tRNA of the invention includes at least about, e.g., a45%, a 50%, a 60%, a 75%, an 80%, or a 90% or more suppressionefficiency in the presence of a cognate synthetase in response to aselector codon as compared to the suppression efficiency of an O-tRNAcomprising or encoded by a polynucleotide sequence as set forth in thesequence listings (e.g., SEQ ID NO: 1) and examples herein. In oneembodiment, the suppression efficiency of the O-RS and the O-tRNAtogether is, e.g., 5 fold, 10 fold, 15 fold, 20 fold, 25 fold or moregreater than the suppression efficiency of the O-tRNA in the absence ofan O-RS. In some aspects, the suppression efficiency of the O-RS and theO-tRNA together is at least 45% of the suppression efficiency of anorthogonal tyrosyl-tRNA synthetase pair derived from Methanococcusjannaschii.

A composition that includes an O-tRNA can optionally include a cell(e.g., a eubacterial cell, such as an E. coli cell and the like, or aeukaryotic cell such as a yeast cell), and/or a translation system.

A cell (e.g., a eubacterial cell or a yeast cell) comprising atranslation system is also provided by the invention, where thetranslation system includes an orthogonal-tRNA (O-tRNA); an orthogonalaminoacyl-tRNA synthetase (O-RS); and, a phenylselenocysteine unnaturalamino acid. Typically, the O-RS preferentially aminoacylates the O-tRNAover any endogenous tRNA with the unnatural amino acid, where the O-RShas a bias for the O-tRNA, and where the ratio of O-tRNA charged withthe unnatural amino acid to the endogenous tRNA charged with theunnatural amino acid is greater than 1:1, and more preferably where theO-RS charges the O-tRNA exclusively or nearly exclusively. The O-tRNArecognizes the first selector codon, and the O-RS preferentiallyaminoacylates the O-tRNA with an unnatural amino acid. In oneembodiment, the O-tRNA comprises or is encoded by a polynucleotidesequence as set forth in SEQ ID NO: 1, or a complementary polynucleotidesequence thereof. In one embodiment, the O-RS comprises an amino acidsequence as set forth in SEQ ID NO: 4, 6, 8 or 10, and conservativevariations thereof.

A cell of the invention can optionally further comprise an additionaldifferent O-tRNA/O-RS pair and a second unnatural amino acid, e.g.,where this O-tRNA recognizes a second selector codon and this O-RSpreferentially aminoacylates the corresponding O-tRNA with the secondunnatural amino acid, where the second amino acid is different from thefirst unnatural amino acid. Optionally, a cell of the invention includesa nucleic acid that comprises a polynucleotide that encodes apolypeptide of interest, where the polynucleotide comprises a selectorcodon that is recognized by the O-tRNA.

In certain embodiments, a cell of the invention is a eubacterial cell(such as E. coli), that includes an orthogonal-tRNA (O-tRNA), anorthogonal aminoacyl-tRNA synthetase (O-RS), an unnatural amino acid,and a nucleic acid that comprises a polynucleotide that encodes apolypeptide of interest, where the polynucleotide comprises the selectorcodon that is recognized by the O-tRNA. In certain embodiments of theinvention, the O-RS preferentially aminoacylates the O-tRNA with theunnatural amino acid with an efficiency that is greater than theefficiency with which the O-RS aminoacylates any endogenous tRNA.

In certain embodiments of the invention, an O-tRNA of the inventioncomprises or is encoded by a polynucleotide sequence as set forth in thesequence listings (e.g., SEQ ID NO: 1) or examples herein, or acomplementary polynucleotide sequence thereof. In certain embodiments ofthe invention, an O-RS comprises an amino acid sequence as set forth inthe sequence listings, or a conservative variation thereof. In oneembodiment, the O-RS or a portion thereof is encoded by a polynucleotidesequence encoding an amino acid as set forth in the sequence listings orexamples herein, or a complementary polynucleotide sequence thereof.

The O-tRNA and/or the O-RS of the invention can be derived from any of avariety of organisms (e.g., eukaryotic and/or non-eukaryotic organisms).

Polynucleotides are also a feature of the invention. A polynucleotide ofthe invention (e.g., SEQ ID NO: 5, 7 or 9) includes an artificial (e.g.,man-made, and not naturally occurring) polynucleotide comprising anucleotide sequence encoding a polypeptide as set forth in the sequencelistings herein, and/or is complementary to or that polynucleotidesequence. A polynucleotide of the invention can also include a nucleicacid that hybridizes to a polynucleotide described above, under highlystringent conditions, over substantially the entire length of thenucleic acid. A polynucleotide of the invention also includes apolynucleotide that is, e.g., at least 75%, at least 80%, at least 90%,at least 95%, at least 98% or more identical to that of a naturallyoccurring tRNA or corresponding coding nucleic acid (but apolynucleotide of the invention is other than a naturally occurring tRNAor corresponding coding nucleic acid), where the tRNA recognizes aselector codon, e.g., a four base codon. Artificial polynucleotides thatare, e.g., at least 80%, at least 90%, at least 95%, at least 98% ormore identical to any of the above and/or a polynucleotide comprising aconservative variation of any the above, are also included inpolynucleotides of the invention.

Vectors comprising a polynucleotide of the invention are also a featureof the invention. For example, a vector of the invention can include aplasmid, a cosmid, a phage, a virus, an expression vector, and/or thelike. A cell comprising a vector of the invention is also a feature ofthe invention.

Methods of producing components of an O-tRNA/O-RS pair are also featuresof the invention. Components produced by these methods are also afeature of the invention. For example, methods of producing at least onetRNA that is orthogonal to a cell (O-tRNA) include generating a libraryof mutant tRNAs; mutating an anticodon loop of each member of thelibrary of mutant tRNAs to allow recognition of a selector codon,thereby providing a library of potential O-tRNAs, and subjecting tonegative selection a first population of cells of a first species, wherethe cells comprise a member of the library of potential O-tRNAs. Thenegative selection eliminates cells that comprise a member of thelibrary of potential O-tRNAs that is aminoacylated by an aminoacyl-tRNAsynthetase (RS) that is endogenous to the cell. This provides a pool oftRNAs that are orthogonal to the cell of the first species, therebyproviding at least one O-tRNA. An O-tRNA produced by the methods of theinvention is also provided.

In certain embodiments, the methods further comprise subjecting topositive selection a second population of cells of the first species,where the cells comprise a member of the pool of tRNAs that areorthogonal to the cell of the first species, a cognate aminoacyl-tRNAsynthetase, and a positive selection marker. Using the positiveselection, cells are selected or screened for those cells that comprisea member of the pool of tRNAs that is aminoacylated by the cognateaminoacyl-tRNA synthetase and that shows a desired response in thepresence of the positive selection marker, thereby providing an O-tRNA.In certain embodiments, the second population of cells comprise cellsthat were not eliminated by the negative selection.

Methods for identifying an orthogonal-aminoacyl-tRNA synthetase thatcharges an O-tRNA with an unnatural amino acid are also provided. Forexample, methods include subjecting a population of cells of a firstspecies to a selection, where the cells each comprise: 1) a member of aplurality of aminoacyl-tRNA synthetases (RSs), (e.g., the plurality ofRSs can include mutant RSs, RSs derived from a species other than afirst species or both mutant RSs and RSs derived from a species otherthan a first species); 2) the orthogonal-tRNA (O-tRNA) (e.g., from oneor more species); and 3) a polynucleotide that encodes a positiveselection marker and comprises at least one selector codon.

Cells (e.g., a host cell) are selected or screened for those that showan enhancement in suppression efficiency compared to cells lacking orhaving a reduced amount of the member of the plurality of RSs. Theseselected/screened cells comprise an active RS that aminoacylates theO-tRNA. An orthogonal aminoacyl-tRNA synthetase identified by the methodis also a feature of the invention.

Methods of producing a protein in a cell (e.g., in a eubacterial cellsuch as an E. coli cell or the like, or in a yeast cell) having theunnatural amino acid at a selected position are also a feature of theinvention. For example, a method includes growing, in an appropriatemedium, a cell, where the cell comprises a nucleic acid that comprisesat least one selector codon and encodes a protein, providing theunnatural amino acid, and incorporating the unnatural amino acid intothe specified position in the protein during translation of the nucleicacid with the at least one selector codon, thereby producing theprotein. The cell further comprises: an orthogonal-tRNA (O-tRNA) thatfunctions in the cell and recognizes the selector codon; and, anorthogonal aminoacyl-tRNA synthetase (O-RS) that preferentiallyaminoacylates the O-tRNA with the unnatural amino acid. A proteinproduced by this method is also a feature of the invention.

The invention also provides compositions that include proteins, wherethe proteins comprise phenylselenocysteine. In certain embodiments, theprotein comprises an amino acid sequence that is at least 75% identicalto that of a known protein, e.g., human growth hormone, a therapeuticprotein, a diagnostic protein, an industrial enzyme, or portion thereof.Optionally, the composition comprises a pharmaceutically acceptablecarrier.

Nucleic Acid and Polypeptide Sequences and Variants

As described herein, the invention provides polynucleotide sequencesencoding, e.g., O-tRNAs and O-RSs, and polypeptide amino acid sequences,e.g., O-RSs, and, e.g., compositions, systems and methods comprisingsaid polynucleotide or polypeptide sequences. Examples of saidsequences, e.g., O-tRNA and O-RS amino acid and nucleotide sequences aredisclosed herein (see FIG. 2). However, one of skill in the art willappreciate that the invention is not limited to those sequencesspecifically recited herein, e.g., in the Examples and sequence listing.One of skill will appreciate that the invention also provides manyrelated sequences with the functions described herein, e.g.,polynucleotides and polypeptides encoding conservative variants of anO-RS disclosed herein.

The construction and analysis of orthogonal synthetase species (O-RS)that are able to aminoacylate a cognate O-tRNA with phenylselenocysteineare described in Example 1. This Example describes the construction andanalysis of the O-RS species that are able to incorporate the unnaturalamino acid phenylselenocysteine.

The invention provides polypeptides (O-RSs) and polynucleotides, e.g.,O-tRNA, polynucleotides that encode O-RSs or portions thereof,oligonucleotides used to isolate aminoacyl-tRNA synthetase clones, etc.Polynucleotides of the invention include those that encode proteins orpolypeptides of interest of the invention with one or more selectorcodon. In addition, polynucleotides of the invention include, e.g., apolynucleotide comprising a nucleotide sequence as set forth in SEQ IDNO: 5, 7 or 9, and a polynucleotide that is complementary to or thatencodes a polynucleotide sequence thereof. A polynucleotide of theinvention also includes any polynucleotide that encodes an O-RS aminoacid sequence comprising SEQ ID NO: 4, 6 or 8. Similarly, an artificialnucleic acid that hybridizes to a polynucleotide indicated above underhighly stringent conditions over substantially the entire length of thenucleic acid (and is other than a naturally occurring polynucleotide) isa polynucleotide of the invention. In one embodiment, a compositionincludes a polypeptide of the invention and an excipient (e.g., buffer,water, pharmaceutically acceptable excipient, etc.). The invention alsoprovides an antibody or antisera specifically immunoreactive with apolypeptide of the invention. An artificial polynucleotide is apolynucleotide that is man made and is not naturally occurring.

A polynucleotide of the invention also includes an artificialpolynucleotide that is, e.g., at least 75%, at least 80%, at least 90%,at least 95%, at least 98% or more identical to that of a naturallyoccurring tRNA, (but is other than a naturally occurring tRNA). Apolynucleotide also includes an artificial polynucleotide that is, e.g.,at least 75%, at least 80%, at least 90%, at least 95%, at least 98% ormore identical (but not 100% identical) to that of a naturally occurringtRNA.

In certain embodiments, a vector (e.g., a plasmid, a cosmid, a phage, avirus, etc.) comprises a polynucleotide of the invention. In oneembodiment, the vector is an expression vector. In another embodiment,the expression vector includes a promoter operably linked to one or moreof the polynucleotides of the invention. In another embodiment, a cellcomprises a vector that includes a polynucleotide of the invention.

One of skill will also appreciate that many variants of the disclosedsequences are included in the invention. For example, conservativevariations of the disclosed sequences that yield a functionallyidentical sequence are included in the invention. Variants of thenucleic acid polynucleotide sequences, wherein the variants hybridize toat least one disclosed sequence, are considered to be included in theinvention. Unique subsequences of the sequences disclosed herein, asdetermined by, e.g., standard sequence comparison techniques, are alsoincluded in the invention.

Conservative Variations

Owing to the degeneracy of the genetic code, “silent substitutions”(i.e., substitutions in a nucleic acid sequence which do not result inan alteration in an encoded polypeptide) are an implied feature of everynucleic acid sequence that encodes an amino acid sequence. Similarly,“conservative amino acid substitutions,” where one or a limited numberof amino acids in an amino acid sequence are substituted with differentamino acids with highly similar properties, are also readily identifiedas being highly similar to a disclosed construct. Such conservativevariations of each disclosed sequence are a feature of the presentinvention.

“Conservative variations” of a particular nucleic acid sequence refersto those nucleic acids which encode identical or essentially identicalamino acid sequences, or, where the nucleic acid does not encode anamino acid sequence, to essentially identical sequences. One of skillwill recognize that individual substitutions, deletions or additionswhich alter, add or delete a single amino acid or a small percentage ofamino acids (typically less than 5%, more typically less than 4%, 2% or1%) in an encoded sequence are “conservatively modified variations”where the alterations result in the deletion of an amino acid, additionof an amino acid, or substitution of an amino acid with a chemicallysimilar amino acid. Thus, “conservative variations” of a listedpolypeptide sequence of the present invention include substitutions of asmall percentage, typically less than 5%, more typically less than 2% or1%, of the amino acids of the polypeptide sequence, with an amino acidof the same conservative substitution group. Finally, the addition ofsequences which do not alter the encoded activity of a nucleic acidmolecule, such as the addition of a non-functional sequence, is aconservative variation of the basic nucleic acid.

Conservative substitution tables providing functionally similar aminoacids are well known in the art, where one amino acid residue issubstituted for another amino acid residue having similar chemicalproperties (e.g., aromatic side chains or positively charged sidechains), and therefore does not substantially change the functionalproperties of the polypeptide molecule. The following sets forth examplegroups that contain natural amino acids of like chemical properties,where substitutions within a group is a “conservative substitution”.

Conservative Amino Acid Substitutions Nonpolar and/or Polar, PositivelyNegatively Aliphatic Uncharged Aromatic Charged Charged Side Chains SideChains Side Chains Side Chains Side Chains Glycine Serine PhenylalanineLysine Aspartate Alanine Threonine Tyrosine Arginine Glutamate ValineCysteine Tryptophan Histidine Leucine Methionine Isoleucine AsparagineProline Glutamine

Nucleic Acid Hybridization

Comparative hybridization can be used to identify nucleic acids of theinvention, including conservative variations of nucleic acids of theinvention, and this comparative hybridization method is a preferredmethod of distinguishing nucleic acids of the invention. In addition,target nucleic acids which hybridize to a nucleic acid represented bySEQ ID NO: 5, 7, 9 or 11, under high, ultra-high and ultra-ultra highstringency conditions are a feature of the invention. Examples of suchnucleic acids include those with one or a few silent or conservativenucleic acid substitutions as compared to a given nucleic acid sequence.

A test nucleic acid is said to specifically hybridize to a probe nucleicacid when it hybridizes at least 50% as well to the probe as to theperfectly matched complementary target, i.e., with a signal to noiseratio at least half as high as hybridization of the probe to the targetunder conditions in which the perfectly matched probe binds to theperfectly matched complementary target with a signal to noise ratio thatis at least about 5×-10× as high as that observed for hybridization toany of the unmatched target nucleic acids.

Nucleic acids “hybridize” when they associate, typically in solution.Nucleic acids hybridize due to a variety of well characterizedphysico-chemical forces, such as hydrogen bonding, solvent exclusion,base stacking and the like. An extensive guide to the hybridization ofnucleic acids is found in Tijssen (1993) Laboratory Techniques inBiochemistry and Molecular Biology—Hybridization with Nucleic AcidProbes, Part I, Chapter 2, “Overview of Principles of Hybridization andthe Strategy of Nucleic Acid Probe Assays,” (Elsevier, New York), aswell as in Current Protocols In Molecular Biology, Ausubel et al., eds.,Current Protocols, a joint venture between Greene Publishing Associates,Inc. and John Wiley & Sons, Inc., (supplemented through 2004); and Hamesand Higgins (1995), Gene Probes 1 and Gene Probes 2, both from IRL Pressat Oxford University Press, Oxford, England, provide details on thesynthesis, labeling, detection and quantification of DNA and RNA,including oligonucleotides.

An example of stringent hybridization conditions for hybridization ofcomplementary nucleic acids which have more than 100 complementaryresidues on a filter in a Southern or northern blot is 50% formalin with1 mg of heparin at 42° C., with the hybridization being carried outovernight. An example of stringent wash conditions is a 0.2×SSC wash at65° C. for 15 minutes (see, Sambrook et al. for a description of SSCbuffer; Sambrook et al., Molecular Cloning—A Laboratory Manual (3rdEd.), Vol. 1-3, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.,2001). Often the high stringency wash is preceded by a low stringencywash to remove background probe signal. An example low stringency washis 2×SSC at 40° C. for 15 minutes. In general, a signal to noise ratioof 5× (or higher) than that observed for an unrelated probe in theparticular hybridization assay indicates detection of a specifichybridization.

“Stringent hybridization wash conditions” in the context of nucleic acidhybridization experiments such as Southern and northern hybridizationsare sequence dependent, and are different under different environmentalparameters. An extensive guide to the hybridization of nucleic acids isfound in Tijssen (1993) Laboratory Techniques in Biochemistry andMolecular Biology—Hybridization with Nucleic Acid Probes, Part I,Chapter 2, “Overview of Principles of Hybridization and the Strategy ofNucleic Acid Probe Assays,” (Elsevier, New York); and in Hames andHiggins (1995), Gene Probes 1 and Gene Probes 2, both from IRL Press atOxford University Press, Oxford, England. Stringent hybridization andwash conditions can easily be determined empirically for any testnucleic acid. For example, in determining stringent hybridization andwash conditions, the hybridization and wash conditions are graduallyincreased (e.g., by increasing temperature, decreasing saltconcentration, increasing detergent concentration and/or increasing theconcentration of organic solvents such as formalin in the hybridizationor wash), until a selected set of criteria are met. For example, inhighly stringent hybridization and wash conditions, the hybridizationand wash conditions are gradually increased until a probe binds to aperfectly matched complementary target with a signal to noise ratio thatis at least 5× as high as that observed for hybridization of the probeto an unmatched target.

“Very stringent” conditions are selected to be equal to the thermalmelting point (T_(m)) for a particular probe. The T_(m) is thetemperature (under defined ionic strength and pH) at which 50% of thetest sequence hybridizes to a perfectly matched probe. For the purposesof the present invention, generally, “highly stringent” hybridizationand wash conditions are selected to be about 5° C. lower than the T_(m)for the specific sequence at a defined ionic strength and pH.

“Ultra high-stringency” hybridization and wash conditions are those inwhich the stringency of hybridization and wash conditions are increaseduntil the signal to noise ratio for binding of the probe to theperfectly matched complementary target nucleic acid is at least 10× ashigh as that observed for hybridization to any of the unmatched targetnucleic acids. A target nucleic acid which hybridizes to a probe undersuch conditions, with a signal to noise ratio of at least ½ that of theperfectly matched complementary target nucleic acid is said to bind tothe probe under ultra-high stringency conditions.

Similarly, even higher levels of stringency can be determined bygradually increasing the hybridization and/or wash conditions of therelevant hybridization assay. For example, those in which the stringencyof hybridization and wash conditions are increased until the signal tonoise ratio for binding of the probe to the perfectly matchedcomplementary target nucleic acid is at least 10×, 20×, 50×, 100×, or500× or more as high as that observed for hybridization to any of theunmatched target nucleic acids. A target nucleic acid which hybridizesto a probe under such conditions, with a signal to noise ratio of atleast ½ that of the perfectly matched complementary target nucleic acidis said to bind to the probe under ultra-ultra-high stringencyconditions.

Nucleic acids which do not hybridize to each other under stringentconditions are still substantially identical if the polypeptides whichthey encode are substantially identical. This occurs, e.g., when a copyof a nucleic acid is created using the maximum codon degeneracypermitted by the genetic code.

Unique Subsequences

In some aspects, the invention provides a nucleic acid that comprises aunique subsequence in a nucleic acid selected from the sequences ofO-tRNAs and O-RSs disclosed herein. The unique subsequence is unique ascompared to a nucleic acid corresponding to any known O-tRNA or O-RSnucleic acid sequence. Alignment can be performed using, e.g., BLAST setto default parameters. Any unique subsequence is useful, e.g., as aprobe to identify the nucleic acids of the invention or related nucleicacids.

Similarly, the invention includes a polypeptide which comprises a uniquesubsequence in a polypeptide selected from the sequences of O-RSsdisclosed herein. Here, the unique subsequence is unique as compared toa polypeptide corresponding to any of known polypeptide sequence.

The invention also provides for target nucleic acids which hybridizesunder stringent conditions to a unique coding oligonucleotide whichencodes a unique subsequence in a polypeptide selected from thesequences of O-RSs wherein the unique subsequence is unique as comparedto a polypeptide corresponding to any of the control polypeptides (e.g.,parental sequences from which synthetases of the invention were derived,e.g., by mutation). Unique sequences are determined as noted above.

Sequence Comparison, Identity, and Homology

The terms “identical” or “percent identity,” in the context of two ormore nucleic acid or polypeptide sequences, refer to two or moresequences or subsequences that are the same or have a specifiedpercentage of amino acid residues or nucleotides that are the same, whencompared and aligned for maximum correspondence, as measured using oneof the sequence comparison algorithms described below (or otheralgorithms available to persons of skill) or by visual inspection.

The phrase “substantially identical,” in the context of two nucleicacids or polypeptides (e.g., DNAs encoding an O-tRNA or O-RS, or theamino acid sequence of an O-RS) refers to two or more sequences orsubsequences that have at least about 60%, about 80%, about 90-95%,about 98%, about 99% or more nucleotide or amino acid residue identity,when compared and aligned for maximum correspondence, as measured usinga sequence comparison algorithm or by visual inspection. Such“substantially identical” sequences are typically considered to be“homologous,” without reference to actual ancestry. Preferably, the“substantial identity” exists over a region of the sequences that is atleast about 50 residues in length, more preferably over a region of atleast about 100 residues, and most preferably, the sequences aresubstantially identical over at least about 150 residues, or over thefull length of the two sequences to be compared.

Proteins and/or protein sequences are “homologous” when they arederived, naturally or artificially, from a common ancestral protein orprotein sequence. Similarly, nucleic acids and/or nucleic acid sequencesare homologous when they are derived, naturally or artificially, from acommon ancestral nucleic acid or nucleic acid sequence. For example, anynaturally occurring nucleic acid can be modified by any availablemutagenesis method to include one or more selector codon. Whenexpressed, this mutagenized nucleic acid encodes a polypeptidecomprising one or more unnatural amino acid. The mutation process can,of course, additionally alter one or more standard codon, therebychanging one or more standard amino acid in the resulting mutant proteinas well. Homology is generally inferred from sequence similarity betweentwo or more nucleic acids or proteins (or sequences thereof). Theprecise percentage of similarity between sequences that is useful inestablishing homology varies with the nucleic acid and protein at issue,but as little as 25% sequence similarity is routinely used to establishhomology. Higher levels of sequence similarity, e.g., 30%, 40%, 50%,60%, 70%, 80%, 90%, 95%, or 99% or more, can also be used to establishhomology. Methods for determining sequence similarity percentages (e.g.,BLASTP and BLASTN using default parameters) are described herein and aregenerally available.

For sequence comparison and homology determination, typically onesequence acts as a reference sequence to which test sequences arecompared. When using a sequence comparison algorithm, test and referencesequences are input into a computer, subsequence coordinates aredesignated, if necessary, and sequence algorithm program parameters aredesignated. The sequence comparison algorithm then calculates thepercent sequence identity for the test sequence(s) relative to thereference sequence, based on the designated program parameters.

Optimal alignment of sequences for comparison can be conducted, e.g., bythe local homology algorithm of Smith and Waterman, Adv. Appl. Math.2:482 (1981), by the homology alignment algorithm of Needleman andWunsch, J. Mol. Biol. 48:443 (1970), by the search for similarity methodof Pearson and Lipman, Proc. Nat'l. Acad. Sci. USA 85:2444 (1988), bycomputerized implementations of these algorithms (GAP, BESTFIT, FASTA,and TFASTA in the Wisconsin Genetics Software Package, Genetics ComputerGroup, 575 Science Dr., Madison, Wis.), or by visual inspection (seegenerally Current Protocols In Molecular Biology, Ausubel et al., eds.,Current Protocols, a joint venture between Greene Publishing Associates,Inc. and John Wiley & Sons, Inc., (supplemented through 2004).

One example of an algorithm that is suitable for determining percentsequence identity and sequence similarity is the BLAST algorithm, whichis described in Altschul et al., J. Mol. Biol., 215:403-410 (1990).Software for performing BLAST analyses is publicly available through theNational Center for Biotechnology Information (see the NCBI website).This algorithm involves first identifying high scoring sequence pairs(HSPs) by identifying short words of length W in the query sequence,which either match or satisfy some positive-valued threshold score Twhen aligned with a word of the same length in a database sequence. T isreferred to as the neighborhood word score threshold (Altschul et al.,(1990) J. Mol. Biol., 215:403-410). These initial neighborhood word hitsact as seeds for initiating searches to find longer HSPs containingthem. The word hits are then extended in both directions along eachsequence for as far as the cumulative alignment score can be increased.Cumulative scores are calculated using, for nucleotide sequences, theparameters M (reward score for a pair of matching residues; always >0)and N (penalty score for mismatching residues; always <0). For aminoacid sequences, a scoring matrix is used to calculate the cumulativescore. Extension of the word hits in each direction are halted when: thecumulative alignment score falls off by the quantity X from its maximumachieved value; the cumulative score goes to zero or below, due to theaccumulation of one or more negative-scoring residue alignments; or theend of either sequence is reached. The BLAST algorithm parameters W, T,and X determine the sensitivity and speed of the alignment. The BLASTNprogram (for nucleotide sequences) uses as defaults a wordlength (W) of11, an expectation (E) of 10, a cutoff of 100, M=5, N=−4, and acomparison of both strands. For amino acid sequences, the BLASTP programuses as defaults a wordlength (W) of 3, an expectation (E) of 10, andthe BLOSUM62 scoring matrix (see Henikoff and Henikoff (1989) Proc.Natl. Acad. Sci. USA 89:10915).

In addition to calculating percent sequence identity, the BLASTalgorithm also performs a statistical analysis of the similarity betweentwo sequences (see, e.g., Karlin and Altschul, Proc. Nat'l. Acad. Sci.USA 90:5873-5787 (1993)). One measure of similarity provided by theBLAST algorithm is the smallest sum probability (P(N)), which providesan indication of the probability by which a match between two nucleotideor amino acid sequences would occur by chance. For example, a nucleicacid is considered similar to a reference sequence if the smallest sumprobability in a comparison of the test nucleic acid to the referencenucleic acid is less than about 0.1, more preferably less than about0.01, and most preferably less than about 0.001.

Mutagenesis and Other Molecular Biology Techniques

Polynucleotide and polypeptides of the invention and used in theinvention can be manipulated using molecular biological techniques.General texts which describe molecular biological techniques includeBerger and Kimmel, Guide to Molecular Cloning Techniques, Methods inEnzymology, volume 152 (1987), Academic Press, Inc., San Diego, Calif.;Sambrook et al., Molecular Cloning—A Laboratory Manual (3rd Ed.), Vol.1-3, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., 2001 andCurrent Protocols In Molecular Biology, Ausubel et al., eds., CurrentProtocols, a joint venture between Greene Publishing Associates, Inc.and John Wiley & Sons, Inc., (supplemented through 2004)). These textsdescribe mutagenesis, the use of vectors, promoters and many otherrelevant topics related to, e.g., the generation of genes that includeselector codons for production of proteins that include unnatural aminoacids, orthogonal tRNAs, orthogonal synthetases, and pairs thereof.

Various types of mutagenesis are used in the invention, e.g., to mutatetRNA molecules, to produce libraries of tRNAs, to produce libraries ofsynthetases, to insert selector codons that encode an unnatural aminoacids in a protein or polypeptide of interest. They include but are notlimited to site-directed, random point mutagenesis, homologousrecombination, DNA shuffling or other recursive mutagenesis methods,chimeric construction, mutagenesis using uracil containing templates,oligonucleotide-directed mutagenesis, phosphorothioate-modified DNAmutagenesis, mutagenesis using gapped duplex DNA or the like, or anycombination thereof. Additional suitable methods include point mismatchrepair, mutagenesis using repair-deficient host strains,restriction-selection and restriction-purification, deletionmutagenesis, mutagenesis by total gene synthesis, double-strand breakrepair, and the like. Mutagenesis, e.g., involving chimeric constructs,is also included in the present invention. In one embodiment,mutagenesis can be guided by known information of the naturallyoccurring molecule or altered or mutated naturally occurring molecule,e.g., sequence, sequence comparisons, physical properties, crystalstructure or the like.

Host cells are genetically engineered (e.g., transformed, transduced ortransfected) with the polynucleotides of the invention or constructswhich include a polynucleotide of the invention, e.g., a vector of theinvention, which can be, for example, a cloning vector or an expressionvector. For example, the coding regions for the orthogonal tRNA, theorthogonal tRNA synthetase, and the protein to be derivatized areoperably linked to gene expression control elements that are functionalin the desired host cell. Typical vectors contain transcription andtranslation terminators, transcription and translation initiationsequences, and promoters useful for regulation of the expression of theparticular target nucleic acid. The vectors optionally comprise genericexpression cassettes containing at least one independent terminatorsequence, sequences permitting replication of the cassette ineukaryotes, or prokaryotes, or both (e.g., shuttle vectors) andselection markers for both prokaryotic and eukaryotic systems. Vectorsare suitable for replication and/or integration in prokaryotes,eukaryotes, or preferably both. See Giliman and Smith, Gene 8:81 (1979);Roberts et al., Nature, 328:731 (1987); Schneider et al., Protein Expr.Purif. 6435:10 (1995); Current Protocols In Molecular Biology, Ausubelet al., Eds., Current Protocols, a joint venture between GreenePublishing Associates, Inc. and John Wiley & Sons, Inc., (supplementedthrough 2004); Sambrook et al., Molecular Cloning—A Laboratory Manual(3rd Ed.), Vol. 1-3, Cold Spring Harbor Laboratory, Cold Spring Harbor,New York, 2001; and Berger and Kimmel, Guide to Molecular CloningTechniques, Methods in Enzymology, volume 152 (1987), Academic Press,Inc., San Diego, Calif. The vector can be, for example, in the form of aplasmid, a bacterium, a virus, a naked polynucleotide, or a conjugatedpolynucleotide. The vectors are introduced into cells and/ormicroorganisms by standard methods including electroporation (Fromm etal., Proc. Natl. Acad. Sci. USA 82, 5824 (1985), infection by viralvectors, high velocity ballistic penetration by small particles with thenucleic acid either within the matrix of small beads or particles, or onthe surface (Klein et al., Nature 327, 70-73 (1987)), and/or the like.

A highly efficient and versatile single plasmid system was developed forsite-specific incorporation of unnatural amino acids into proteins inresponse to the amber stop codon (UAG) in E. coli. In the new system,the pair of M. jannaschii suppressor tRNAtyr(CUA) and tyrosyl-tRNAsynthetase are encoded in a single plasmid, which is compatible withmost E. coli expression vectors. Monocistronic tRNA operon under controlof proK promoter and terminator was constructed for optimal secondarystructure and tRNA processing. Introduction of a mutated form of glnSpromoter for the synthetase resulted in a significant increase in bothsuppression efficiency and fidelity. Increases in suppression efficiencywere also obtained by multiple copies of tRNA gene as well as by aspecific mutation (D286R) on the synthetase (Kobayashi et al.,“Structural basis for orthogonal tRNA specificities of tyrosyl-tRNAsynthetases for genetic code expansion,” Nat. Struct. Biol.,10(6):425-432 [2003]). The generality of the optimized system was alsodemonstrated by highly efficient and accurate incorporation of severaldifferent unnatural amino acids, whose unique utilities in studyingprotein function and structure were previously proven.

A catalogue of Bacteria and Bacteriophages useful for cloning isprovided, e.g., by the ATCC, e.g., The ATCC Catalogue of Bacteria andBacteriophage (1996) Gherna et al. (eds) published by the ATCC.Additional basic procedures for sequencing, cloning and other aspects ofmolecular biology and underlying theoretical considerations are alsofound in Sambrook et al., Molecular Cloning—A Laboratory Manual (3rdEd.), Vol. 1-3, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.,2001; Current Protocols In Molecular Biology, Ausubel et al., eds.,Current Protocols, a joint venture between Greene Publishing Associates,Inc. and John Wiley & Sons, Inc., (supplemented through 2004); and inWatson et al. (1992) Recombinant DNA Second Edition Scientific AmericanBooks, NY. In addition, essentially any nucleic acid (and virtually anylabeled nucleic acid, whether standard or non-standard) can be custom orstandard ordered from any of a variety of commercial sources, such asthe Midland Certified Reagent Company (Midland, Tex.), The GreatAmerican Gene Company (Ramona, Calif.), ExpressGen Inc. (Chicago, Ill.),Operon Technologies Inc. (Alameda, Calif.) and many others.

The engineered host cells can be cultured in conventional nutrient mediamodified as appropriate for such activities as, for example, screeningsteps, activating promoters or selecting transformants. These cells canoptionally be cultured into transgenic organisms. Other usefulreferences, e.g. for cell isolation and culture (e.g., for subsequentnucleic acid isolation) include Freshney (1994) Culture of Animal Cells,a Manual of Basic Technique, third edition, Wiley-Liss, New York and thereferences cited therein; Payne et al. (1992) Plant Cell and TissueCulture in Liquid Systems John Wiley & Sons, Inc. New York, N.Y.;Gamborg and Phillips (eds) (1995) Plant Cell, Tissue and Organ Culture;Fundamental Methods Springer Lab Manual, Springer-Verlag (BerlinHeidelberg New York) and Atlas and Parks (eds) The Handbook ofMicrobiological Media (1993) CRC Press, Boca Raton, Fla.

Proteins and Polypeptides of Interest

Methods of producing a protein in a cell with an unnatural amino acid ata specified position are also a feature of the invention. For example, amethod includes growing, in an appropriate medium, the cell, where thecell comprises a nucleic acid that comprises at least one selector codonand encodes a protein; and, providing the unnatural amino acid; wherethe cell further comprises: an orthogonal-tRNA (O-tRNA) that functionsin the cell and recognizes the selector codon; and, an orthogonalaminoacyl-tRNA synthetase (O-RS) that preferentially aminoacylates theO-tRNA with the unnatural amino acid. A protein produced by this methodis also a feature of the invention.

In certain embodiments, the O-RS comprises a bias for the aminoacylationof the cognate O-tRNA over any endogenous tRNA in an expression system.The relative ratio between O-tRNA and endogenous tRNA that is charged bythe O-RS, when the O-tRNA and O-RS are present at equal molarconcentrations, is greater than 1:1, preferably at least about 2:1, morepreferably 5:1, still more preferably 10:1, yet more preferably 20:1,still more preferably 50:1, yet more preferably 75:1, still morepreferably 95:1, 98:1, 99:1, 100:1, 500:1, 1,000:1, 5,000:1 or higher.

The invention also provides compositions that include proteins, wherethe proteins comprise an unnatural amino acid. In certain embodiments,the protein comprises an amino acid sequence that is at least 75%identical to that of a therapeutic protein, a diagnostic protein, anindustrial enzyme, or portion thereof.

The compositions of the invention and compositions made by the methodsof the invention optionally are in a cell. The O-tRNA/O-RS pairs orindividual components of the invention can then be used in a hostsystem's translation machinery, which results in an unnatural amino acidbeing incorporated into a protein. International Publication Numbers WO2004/094593, filed Apr. 16, 2004, entitled “EXPANDING THE EUKARYOTICGENETIC CODE,” and WO 2002/085923, entitled “IN VIVO INCORPORATION OFUNNATURAL AMINO ACIDS,” describe this process, and are incorporatedherein by reference. For example, when an O-tRNA/O-RS pair is introducedinto a host, e.g., an Escherichia coli cell, the pair leads to the invivo incorporation of an unnatural amino acid such asphenylselenocysteine into a protein in response to a selector codon. Theunnatural amino acid that is added to the system can be a syntheticamino acid, such as a derivative of a phenylalanine or tyrosine, whichcan be exogenously added to the growth medium. Optionally, thecompositions of the present invention can be in an in vitro translationsystem, or in an in vivo system(s).

A cell of the invention provides the ability to synthesize proteins thatcomprise unnatural amino acids in large useful quantities. In someaspects, the composition optionally includes, e.g., at least 10micrograms, at least 50 micrograms, at least 75 micrograms, at least 100micrograms, at least 200 micrograms, at least 250 micrograms, at least500 micrograms, at least 1 milligram, at least 10 milligrams or more ofthe protein that comprises an unnatural amino acid, or an amount thatcan be achieved with in vivo protein production methods (details onrecombinant protein production and purification are provided herein). Inanother aspect, the protein is optionally present in the composition ata concentration of, e.g., at least 10 micrograms of protein per liter,at least 50 micrograms of protein per liter, at least 75 micrograms ofprotein per liter, at least 100 micrograms of protein per liter, atleast 200 micrograms of protein per liter, at least 250 micrograms ofprotein per liter, at least 500 micrograms of protein per liter, atleast 1 milligram of protein per liter, or at least 10 milligrams ofprotein per liter or more, in, e.g., a cell lysate, a buffer, apharmaceutical buffer, or other liquid suspension (e.g., in a volume of,e.g., anywhere from about 1 nL to about 100 L). The production of largequantities (e.g., greater that that typically possible with othermethods, e.g., in vitro translation) of a protein in a cell including atleast one unnatural amino acid is a feature of the invention.

The incorporation of an unnatural amino acid can be done to, e.g.,tailor changes in protein structure and/or function, e.g., to changesize, acidity, nucleophilicity, hydrogen bonding, hydrophobicity,accessibility of protease target sites, target to a moiety (e.g., for aprotein array), incorporation of labels or reactive groups, etc.Proteins that include an unnatural amino acid can have enhanced or evenentirely new catalytic or physical properties. For example, thefollowing properties are optionally modified by inclusion of anunnatural amino acid into a protein: toxicity, biodistribution,structural properties, spectroscopic properties, chemical and/orphotochemical properties, catalytic ability, half-life (e.g., serumhalf-life), ability to react with other molecules, e.g., covalently ornoncovalently, and the like. The compositions including proteins thatinclude at least one unnatural amino acid are useful for, e.g., noveltherapeutics, diagnostics, catalytic enzymes, industrial enzymes,binding proteins (e.g., antibodies), and e.g., the study of proteinstructure and function. See, e.g., Dougherty, (2000) Unnatural AminoAcids as Probes of Protein Structure and Function, Current Opinion inChemical Biology, 4:645-652.

In some aspects of the invention, a composition includes at least oneprotein with at least one, e.g., at least two, at least three, at leastfour, at least five, at least six, at least seven, at least eight, atleast nine, or at least ten or more unnatural amino acids. The unnaturalamino acids can be the same or different, e.g., there can be 1, 2, 3, 4,5, 6, 7, 8, 9, or 10 or more different sites in the protein thatcomprise 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more different unnaturalamino acids. In another aspect, a composition includes a protein with atleast one, but fewer than all, of a particular amino acid present in theprotein is an unnatural amino acid. For a given protein with more thanone unnatural amino acids, the unnatural amino acids can be identical ordifferent (e.g., the protein can include two or more different types ofunnatural amino acids, or can include two of the same unnatural aminoacid). For a given protein with more than two unnatural amino acids, theunnatural amino acids can be the same, different or a combination of amultiple unnatural amino acid of the same kind with at least onedifferent unnatural amino acid.

Essentially any protein (or portion thereof) that includes an unnaturalamino acid (and any corresponding coding nucleic acid, e.g., whichincludes one or more selector codons) can be produced using thecompositions and methods herein. No attempt is made to identify thehundreds of thousands of known proteins, any of which can be modified toinclude one or more unnatural amino acid, e.g., by tailoring anyavailable mutation methods to include one or more appropriate selectorcodon in a relevant translation system. Common sequence repositories forknown proteins include GenBank EMBL, DDBJ and the NCBI. Otherrepositories can easily be identified by searching the internet.

Typically, the proteins are, e.g., at least 60%, at least 70%, at least75%, at least 80%, at least 90%, at least 95%, or at least 99% or moreidentical to any available protein (e.g., a therapeutic protein, adiagnostic protein, an industrial enzyme, or portion thereof, and thelike), and they comprise one or more unnatural amino acid. Examples oftherapeutic, diagnostic, and other proteins that can be modified tocomprise one or more unnatural amino acid can be found, but not limitedto, those in International Publications WO 2004/094593, filed Apr. 16,2004, entitled “EXPANDING THE EUKARYOTIC GENETIC CODE;” and, WO2002/085923, entitled “IN VIVO INCORPORATION OF UNNATURAL AMINO ACIDS.”Examples of therapeutic, diagnostic, and other proteins that can bemodified to comprise one or more unnatural amino acids include, but arenot limited to, e.g., hirudin, human growth hormone, RAS, Alpha-1antitrypsin, Angiostatin, Antihemolytic factor, antibodies (furtherdetails on antibodies are found below), Apolipoprotein, Apoprotein,Atrial natriuretic factor, Atrial natriuretic polypeptide, Atrialpeptides, C-X-C chemokines (e.g., T39765, NAP-2, ENA-78, Gro-a, Gro-b,Gro-c, IP-10, GCP-2, NAP-4, SDF-1, PF4, MIG), Calcitonin, CC chemokines(e.g., Monocyte chemoattractant protein-1, Monocyte chemoattractantprotein-2, Monocyte chemoattractant protein-3, Monocyte inflammatoryprotein-1 alpha, Monocyte inflammatory protein-1 beta, RANTES, I309,R83915, R91733, HCC1, T58847, D31065, T64262), CD40 ligand, C-kitLigand, Collagen, Colony stimulating factor (CSF), Complement factor 5a,Complement inhibitor, Complement receptor 1, cytokines, (e.g.,epithelial Neutrophil Activating Peptide-78, GROα/MGSA, GROβ, GROγ,MIP-1α, MIP-1δ, MCP-1), Epidermal Growth Factor (EGF), Erythropoietin(“EPO”), Exfoliating toxins A and B, Factor IX, Factor VII, Factor VIII,Factor X, Fibroblast Growth Factor (FGF), Fibrinogen, Fibronectin,G-CSF, GM-CSF, Glucocerebrosidase, Gonadotropin, growth factors,Hedgehog proteins (e.g., Sonic, Indian, Desert), Hemoglobin, HepatocyteGrowth Factor (HGF), hirudin, Human serum albumin, Insulin, Insulin-likeGrowth Factor (IGF), interferons (e.g., IFN-α, IFN-β, IFN-γ),interleukins (e.g., IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-8,IL-9, IL-10, IL-11, IL-12, etc.), Keratinocyte Growth Factor (KGF),Lactoferrin, leukemia inhibitory factor, Luciferase, Neurturin,Neutrophil inhibitory factor (NIF), oncostatin M, Osteogenic protein,Parathyroid hormone, PD-ECSF, PDGF, peptide hormones (e.g., Human GrowthHormone), Pleiotropin, Protein A, Protein G, Pyrogenic exotoxins A, B,and C, Relaxin, Renin, SCF, Soluble complement receptor I, Soluble I-CAM1, Soluble interleukin receptors (IL-1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 12,13, 14, 15), Soluble TNF receptor, Somatomedin, Somatostatin,Somatotropin, Streptokinase, Superantigens, i.e., Staphylococcalenterotoxins (SEA, SEB, SEC1, SEC2, SEC3, SED, SEE), superoxidedismutase (SOD), toxic shock syndrome toxin (TSST-1), thymosin alpha 1,Tissue plasminogen activator, tumor necrosis factor beta (TNF beta),tumor necrosis factor receptor (TNFR), tumor necrosis factor-alpha (TNFalpha), vascular Endothelial Growth Factor (VEGEF), urokinase and manyothers.

One class of proteins that can be made using the compositions andmethods for in vivo incorporation of unnatural amino acids describedherein includes transcriptional modulators or a portion thereof. Exampletranscriptional modulators include genes and transcriptional modulatorproteins that modulate cell growth, differentiation, regulation, or thelike. Transcriptional modulators are found in prokaryotes, viruses, andeukaryotes, including fungi, plants, yeasts, insects, and animals,including mammals, providing a wide range of therapeutic targets. Itwill be appreciated that expression and transcriptional activatorsregulate transcription by many mechanisms, e.g., by binding toreceptors, stimulating a signal transduction cascade, regulatingexpression of transcription factors, binding to promoters and enhancers,binding to proteins that bind to promoters and enhancers, unwinding DNA,splicing pre-mRNA, polyadenylating RNA, and degrading RNA.

One class of proteins of the invention (e.g., proteins with one or moreunnatural amino acids) include biologically active proteins such ascytokines, inflammatory molecules, growth factors, their receptors, andoncogene products, e.g., interleukins (e.g., IL-1, IL-2, IL-8, etc.),interferons, FGF, IGF-I, IGF-II, FGF, PDGF, TNF, TGF-α, TGF-β, EGF, KGF,SCF/c-Kit, CD40L/CD40, VLA-4/VCAM-1, ICAM-1/LFA-1, and hyalurin/CD44;signal transduction molecules and corresponding oncogene products, e.g.,Mos, Ras, Raf, and Met; and transcriptional activators and suppressors,e.g., p53, Tat, Fos, Myc, Jun, Myb, Rel, and steroid hormone receptorssuch as those for estrogen, progesterone, testosterone, aldosterone, theLDL receptor ligand and corticosterone.

Enzymes (e.g., industrial enzymes) or portions thereof with at least oneunnatural amino acid are also provided by the invention. Examples ofenzymes include, but are not limited to, e.g., amidases, amino acidracemases, acylases, dehalogenases, dioxygenases, diarylpropaneperoxidases, epimerases, epoxide hydrolases, esterases, isomerases,kinases, glucose isomerases, glycosidases, glycosyl transferases,haloperoxidases, monooxygenases (e.g., p450s), lipases, ligninperoxidases; nitrile hydratases, nitrilases, proteases, phosphatases,subtilisins, transaminase, and nucleases.

Many of these proteins are commercially available (See, e.g., SigmaBioSciences), and the corresponding protein sequences and genes, andtypically, many variants thereof, are well-known (see, e.g., Genbank).Any of them can be modified by the insertion of one or more unnaturalamino acid according to the invention, e.g., to alter the protein withrespect to one or more therapeutic, diagnostic or enzymatic propertiesof interest. Examples of therapeutically relevant properties includeserum half-life, shelf half-life, stability, immunogenicity, therapeuticactivity, detectability (e.g., by the inclusion of reporter groups(e.g., labels or label binding sites) in the unnatural amino acids),reduction of LD₅₀ or other side effects, ability to enter the bodythrough the gastric tract (e.g., oral availability), or the like.Examples of diagnostic properties include shelf half-life, stability,diagnostic activity, detectability, or the like. Examples of relevantenzymatic properties include shelf half-life, stability, enzymaticactivity, production capability, or the like.

A variety of other proteins can also be modified to include one or moreunnatural amino acid using compositions and methods of the invention.For example, the invention can include substituting one or more naturalamino acids in one or more vaccine proteins with an unnatural aminoacid, e.g., in proteins from infectious fungi, e.g., Aspergillus,Candida species; bacteria, particularly E. coli, which serves a modelfor pathogenic bacteria, as well as medically important bacteria such asStaphylococci (e.g., aureus), or Streptococci (e.g., pneumoniae);protozoa such as sporozoa (e.g., Plasmodia), rhizopods (e.g., Entamoeba)and flagellates (Trypanosoma, Leishmania, Trichomonas, Giardia, etc.);viruses such as (+) RNA viruses (examples include Poxviruses e.g.,vaccinia; Picornaviruses, e.g. polio; Togaviruses, e.g., rubella;Flaviviruses, e.g., HCV; and Coronaviruses), (−) RNA viruses (e.g.,Rhabdoviruses, e.g., VSV; Paramyxovimses, e.g., RSV; Orthomyxovimses,e.g., influenza; Bunyaviruses; and Arenaviruses), dsDNA viruses(Reoviruses, for example), RNA to DNA viruses, i.e., Retroviruses, e.g.,HIV and HTLV, and certain DNA to RNA viruses such as Hepatitis B.

Agriculturally related proteins such as insect resistance proteins(e.g., the Cry proteins), starch and lipid production enzymes, plant andinsect toxins, toxin-resistance proteins, Mycotoxin detoxificationproteins, plant growth enzymes (e.g., Ribulose 1,5-BisphosphateCarboxylase/Oxygenase, “RUBISCO”), lipoxygenase (LOX), andPhosphoenolpyruvate (PEP) carboxylase are also suitable targets forunnatural amino acid modification.

In certain embodiments, the protein or polypeptide of interest (orportion thereof) in the methods and/or compositions of the invention isencoded by a nucleic acid. Typically, the nucleic acid comprises atleast one selector codon, at least two selector codons, at least threeselector codons, at least four selector codons, at least five selectorcodons, at least six selector codons, at least seven selector codons, atleast eight selector codons, at least nine selector codons, ten or moreselector codons.

Genes coding for proteins or polypeptides of interest can be mutagenizedusing methods well-known to one of skill in the art and described hereinunder “Mutagenesis and Other Molecular Biology Techniques” to include,e.g., one or more selector codon for the incorporation of an unnaturalamino acid. For example, a nucleic acid for a protein of interest ismutagenized to include one or more selector codon, providing for theinsertion of the one or more unnatural amino acids. The inventionincludes any such variant, e.g., mutant, versions of any protein, e.g.,including at least one unnatural amino acid. Similarly, the inventionalso includes corresponding nucleic acids, i.e., any nucleic acid withone or more selector codon that encodes one or more unnatural aminoacid.

To make a protein that includes an unnatural amino acid, one can usehost cells and organisms that are adapted for the in vivo incorporationof the unnatural amino acid via orthogonal tRNA/RS pairs. Host cells aregenetically engineered (e.g., transformed, transduced or transfected)with one or more vectors that express the orthogonal tRNA, theorthogonal tRNA synthetase, and a vector that encodes the protein to bederivatized. Each of these components can be on the same vector, or eachcan be on a separate vector, or two components can be on one vector andthe third component on a second vector. The vector can be, for example,in the form of a plasmid, a bacterium, a virus, a naked polynucleotide,or a conjugated polynucleotide.

Defining Polypeptides by Immunoreactivity

Because the polypeptides of the invention provide a variety of newpolypeptide sequences (e.g., polypeptides comprising unnatural aminoacids in the case of proteins synthesized in the translation systemsherein, or, e.g., in the case of the novel synthetases, novel sequencesof standard amino acids), the polypeptides also provide new structuralfeatures which can be recognized, e.g., in immunological assays. Thegeneration of antisera, which specifically bind the polypeptides of theinvention, as well as the polypeptides which are bound by such antisera,are a feature of the invention. The term “antibody,” as used herein,includes, but is not limited to a polypeptide substantially encoded byan immunoglobulin gene or immunoglobulin genes, or fragments thereofwhich specifically bind and recognize an analyte (antigen). Examplesinclude polyclonal, monoclonal, chimeric, and single chain antibodies,and the like. Fragments of immunoglobulins, including Fab fragments andfragments produced by an expression library, including phage display,are also included in the term “antibody” as used herein. See, e.g.,Paul, Fundamental Immunology, 4th Ed., 1999, Raven Press, New York, forantibody structure and terminology.

In order to produce antisera for use in an immunoassay, one or more ofthe immunogenic polypeptides is produced and purified as describedherein. For example, recombinant protein can be produced in arecombinant cell. An inbred strain of mice (used in this assay becauseresults are more reproducible due to the virtual genetic identity of themice) is immunized with the immunogenic protein(s) in combination with astandard adjuvant, such as Freund's adjuvant, and a standard mouseimmunization protocol (see, e.g., Harlow and Lane (1988) Antibodies, ALaboratory Manual, Cold Spring Harbor Publications, New York, for astandard description of antibody generation, immunoassay formats andconditions that can be used to determine specific immunoreactivity.Additional details on proteins, antibodies, antisera, etc. can be foundin International Publication Numbers WO 2004/094593, entitled “EXPANDINGTHE EUKARYOTIC GENETIC CODE;” WO 2002/085923, entitled “IN VIVOINCORPORATION OF UNNATURAL AMINO ACIDS;” WO 2004/035605, entitled“GLYCOPROTEIN SYNTHESIS;” and WO 2004/058946, entitled “PROTEIN ARRAYS.”

Use of O-tRNA and O-RS and O-tRNA/O-RS Pairs

The compositions of the invention and compositions made by the methodsof the invention optionally are in a cell. The O-tRNA/O-RS pairs orindividual components of the invention can then be used in a hostsystem's translation machinery, which results in an unnatural amino acidbeing incorporated into a protein. International Publication Number WO2002/085923 by Schultz, et al., entitled “IN VIVO INCORPORATION OFUNNATURAL AMINO ACIDS,” describes this process and is incorporatedherein by reference. For example, when an O-tRNA/O-RS pair is introducedinto a host, e.g., Escherichia coli or yeast, the pair leads to the invivo incorporation of an unnatural amino acid, which can be exogenouslyadded to the growth medium, into a protein, e.g., a myoglobin testprotein or a therapeutic protein, in response to a selector codon, e.g.,an amber nonsense codon. Optionally, the compositions of the inventioncan be in an in vitro translation system, or in a cellular in vivosystem(s). Proteins with the unnatural amino acid can be used in any ofa wide range of applications. For example, the unnatural moietyincorporated into a protein can serve as a target for any of a widerange of modifications, for example, crosslinking with other proteins,with small molecules such as labels or dyes and/or biomolecules. Withthese modifications, incorporation of the unnatural amino acid canresult in improved therapeutic proteins and can be used to alter orimprove the catalytic function of enzymes. In some aspects, theincorporation and subsequent modification of an unnatural amino acid ina protein can facilitate studies on protein structure, interactions withother proteins, and the like.

Kits

Kits are also a feature of the invention. For example, a kit forproducing a protein that comprises at least one unnatural amino acid ina cell is provided, where the kit includes at least one containercontaining a polynucleotide sequence encoding an O-tRNA, and/or anO-tRNA, and/or a polynucleotide sequence encoding an O-RS, and/or anO-RS. In one embodiment, the kit further includes the unnatural aminoacid phenylselenocysteine. In another embodiments, the kit furthercomprises instructional materials for producing the protein and/or ahost cell.

EXAMPLES

The following examples are offered to illustrate, but not to limit theclaimed invention. One of skill will recognize a variety of non-criticalparameters that may be altered without departing from the scope of theclaimed invention. It is understood that the examples and embodimentsdescribed herein are for illustrative purposes only and that variousmodifications or changes in light thereof will be suggested to personsskilled in the art and are to be included within the spirit and purviewof this application and scope of the appended claims.

Example Genetic Selection of Mutant Synthetases Specific forPhenylselenocysteine

Phenylselenocysteine Reactive Chemistry

Selenium is an essential element in both organic synthesis and biology.It has been demonstrated that a phenylseleno moiety can be oxidized inmild conditions, and spontaneous syn elimination of the selenoxideresults in olefin formation. This strategy has been used extensively intotal synthesis.

The incorporation of an unnatural amino acid containing selenium intoproteins would create an attractive target for highly selectiveposttranslational modification of proteins, for example, to produceselectively lipidated proteins. One such unnatural seleno amino acid isphenylselenocysteine (CAS Registry Number 71128-82-0), synonymouslytermed phenylselenide or Se-Phenyl-L-selenocysteine. This structure isprovided in FIG. 1, structure 1. This unnatural amino acid can beobtained, for example, from Sigma®-Aldrich® Co., Inc., catalog number50827.

Oxidative cleavage of a phenylselenocysteine amino acid residue resultsin the α,β-unsaturated amino acid dehydroalanine (see FIG. 1, structure2). Dehydroalanine is found in a number of naturally occurring peptides(Chatterjee et al. (2005), Chemical Reviews 105(2):633-683).Dehydropeptides containing unprotected cysteine residues undergointramolecular stereoselective conjugate addition to produce cycliclanthionines (Chatterjee et al. (2005), Chemical Reviews105(2):633-683). Lanthionines are structures that are found inlantibiotics, a class of post-translationally modified antibiotics(Chatterjee et al. (2005), Chemical Reviews 105(2):633-683).

Michael Addition reactions of the unnatural amino acid dehydroalanine(see FIG. 1, structure 2) can result in proteins with post-translationalmodifications, for example, by reaction with a thio-lipid to generate alipidated protein. More specifically, reaction with thiopalmitic acidresults in palmitoylcysteine (see FIG. 1, structure 3), reaction withfarnesylmercaptan affords farnesylcysteine (see FIG. 1, structure 4),and reaction with malonate produces γ-carboxyglutamic acid (see FIG. 1,structure 6). In addition, reaction with 1-hexadecanethiol results inS-hexadecylcysteine (see FIG. 1, structure 5). AlthoughS-hexadecylcysteine is not a native post-translational modification, thepresence of this residue in protein can have desired properties; forexample, it can result in human serum albumin binding, and higherprotein stability in vivo.

The lipidation of polypeptides by targeted modification ofphenylselenocysteine residues can have desired properties, such asmembrane localization, improved in vivo stability (i.e., improvedhalf-life) and lipid solubility. Furthermore, the lipidated forms ofsome proteins are the biologically active forms.

Production of Proteins Comprising Phenylselenocysteine

Methodologies that allow the systematic addition of unnatural aminoacids to the genetic codes of E. coli, yeast and mammalian cells havebeen previously described. Such methods are based on the evolution of anonsense suppressor tRNA and an aminoacyl-tRNA synthetase (RS) pair thathas the property of orthogonality, defined as the ability to selectivelyincorporate a given amino acid in response to a unique codon withoutcross-reacting with endogenous host tRNAs, aminoacyl-tRNA synthetases,or amino acids.

The present invention provides compositions and methods for the in vivogeneration of polypeptides containing the unnatural amino acidphenylselenocysteine by creation of orthogonal reagents that permit thegenetically programmed translational incorporation of this unnaturalamino acid directly into a growing polypeptide chain. Furthermore, theuse of these orthogonal systems allows the production of largequantities of polypeptides containing the unnatural amino acid (i.e.,much larger quantities of the polypeptides than would be possible usingother synthesis methods). Polypeptides comprising phenylselenocysteinecan be used in targeted modification reactions, for example but notlimited to, the generation of artificially lipidated polypeptidesaccording to the teachings of the present specification. Such modifiedproteins find a variety of uses, including but not limited to improvedtherapeutic agents.

To generate an orthogonal tRNA/aaRS pair that uniquely insertsphenylselenocysteine (FIG. 1., structure 1), a library of active sitemutants of the Methanococcus jannaschii tyrosyl-tRNA synthetase(MjTyrRS), which specifically charges an engineered M. jannaschiinonsense suppressor (MjtRNA^(Tyr) _(CUA)) not recognized by E. colisynthetases, was used. The synthetase library pBK-lib5, as described inWang et al. (2006), Annual Review of Biophysics and BiomolecularStructure 35:225-249, was used in the screening and subjected to aseries of positive and negative selections. Survival in the positiveselection was contingent upon suppression of an amber mutation in thechloramphenicol acetyltransferase (CAT) gene in the presence ofphenylselenocysteine; survival in the negative selection was contingentupon inadequate suppression of amber mutations in a gene encoding thetoxic barnase protein in the absence of phenylselenocysteine. Clonessurvive through both positive and negative rounds of selection only ifthey uniquely incorporate phenylselenocysteine in response to the ambercodon.

Following these selections, clone candidates were identified thatallowed cells harboring the CAT gene with an amber mutation selectorcodon at a permissive site to survive in the presence of chloramphenicolonly in the presence of phenylselenocysteine. In the absence ofphenylselenocysteine, the same cells did not grow in the presence ofchloramphenicol, consistent with efficient phenylselenocysteineincorporation with little to no background survival from incorporationof endogenous amino acids.

Sequencing of the candidate mutant synthetase clones revealed threedifferent synthetase isolates, each of which is capable of functioningin an orthogonal translation system.

-   Clone 1 (PhSeRS-SD): Y32W, L65E, H70G, D158Q, L162S-   Clone 2 (PhSeRS-K4): Y32W, L65H, H70G, F108N, Q109S, D158S, L162E-   Clone 3 (PhSeRS-K5): Y32W, L65H, A67G, H70G, F108K, Q109S, D158E,    L162E

These clones are further summarized in the table below.

Methanococcus jannaschii tyrosyl-tRNA synthetase amino acid sequencesWild-Type and Mutants SEQ ID 32 65 67 70 108 109 158 162 NO: wild- TyrLeu Ala His Phe Gln Asp Leu 2 type Mutant Trp Glu Ala Gly Phe Gln GlnSer 4 Clone 1 (SD) Mutant Trp His Ala Gly Asn Ser Ser Glu 6 Clone 2 (K4)Mutant Trp His Gly Gly Lys Ser Glu Glu 8 Clone 3 (K5)

Of the three mutant synthetases isolated, the clone 2 synthetase(PhSeRS-K4) had the highest activity and specificity. The completenucleotide and amino acid sequences of each of these clones and thecorresponding wild-type species are provided in FIG. 2.

Orthogonal Synthetase Validation

Using the mutant synthetases described herein, a myoglobin model proteincontaining an amber selector codon at position 4 (TAG4) was expressed in1.5-2 mg/L yield with no background expression in the absence ofphenylselenocysteine. This model protein containing phenylselenocysteinewas further subjected to posttranslational modification to produceγ-carboxyglutamic acid (FIG. 1 structure 6), which was confirmed usinghigh-resolution mass-spec.

Two additional model proteins comprising phenylselenocysteine were alsoexpressed in 3-5 mg/L yields using the mutant synthetases and in vivoorthogonal production system described herein. Human growth hormone(hGH) was produced containing phenylselenocysteine. Following productionof the phenylselenocysteine form, the protein was subject tomodification to produce hGH that contains S-hexadecylcysteine. The humanserum albumin binding properties and the in vivo half life of themodified hGH in mice are currently being tested.

The orthogonal reagents described herein were also used to produce greenfluorescent protein (GFP) containing phenylselenocysteine. That form ofthe protein was also subsequently modified to form, alternatively,S-hexadecylcysteine, farnesylcysteine and palmitoylcysteine forms ofGFP. The membrane-targeting properties of these lipidated forms of GFPare currently being tested.

Thus, the present invention provides compositions and methods for thegenetically-programmed production of proteins containing the unnaturalamino acid phenylselenocysteine (see FIG. 1, structure 1). Subsequentoxidative elimination by hydrogen peroxide produces dehydroalanine (seeFIG. 1, structure 2) at the desired position. This dehydroalanine moietycan then be targeted in various secondary modification reactions.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be clear to one skilledin the art from a reading of this disclosure that various changes inform and detail can be made without departing from the true scope of theinvention. All publications, patents, patent applications, and/or otherdocuments cited in this application are incorporated by reference intheir entirety for all purposes to the same extent as if each individualpublication, patent, patent application, and/or other document wereindividually indicated to be incorporated by reference for all purposes.

1. A translation system comprising: (a) a first unnatural amino acidthat is phenylselenocysteine, (b) a first orthogonal aminoacyl-tRNAsynthetase (O-RS), wherein the O-RS is at least 90% identical to SEQ IDNO: 6 and comprises: a Trp at a position corresponding to position 32 ofSEQ ID NO: 6 and a Gly at a position corresponding to position 70 of SEQID NO:6; and, wherein the first O-RS can aminoacylate a reference tRNAcomprising SEQ ID NO: 1 (MjtRNA-Tyr_(CUA)) with thephenylselenocysteine; and (c) a first orthogonal tRNA (O-tRNA), whereinthe O-tRNA is at least 90% identical to SEQ ID NO: 1; and wherein theO-tRNA can be aminoacylated with the phenylselenocysteine by a referenceO-RS comprising SEQ ID NO: 6; wherein said first O-RS functions topreferentially aminoacylate said first O-tRNA with saidphenylselenocysteine.
 2. The translation system of claim 1, wherein saidfirst O-RS functions to preferentially aminoacylate said first O-tRNAwith said phenylselenocysteine with an efficiency that is at least 50%of the efficiency of a reference aminoacyl-tRNA synthetase comprisingthe amino acid sequence of SEQ ID NO: 4, 6 or
 8. 3. The translationsystem of claim 1, wherein said first O-RS comprises an amino acidsequence selected from the group consisting of amino acid sequences setforth in SEQ ID NOS: 4, 6, 8, and conservative variants thereof.
 4. Thetranslation system of claim 1, wherein said first O-tRNA is an ambersuppressor tRNA.
 5. The translation system of claim 1, wherein saidfirst O-tRNA comprises the polynucleotide sequence set forth in SEQ IDNO:
 1. 6. The translation system of claim 1, further comprising anucleic acid encoding a protein of interest, said nucleic acidcomprising at least one selector codon, wherein said selector codon isrecognized by said first O-tRNA.
 7. The translation system of claim 6,further comprising a second O-RS and a second O-tRNA, wherein the secondO-RS functions to preferentially aminoacylate the second O-tRNA with asecond unnatural amino acid that is different from the first unnaturalamino acid, and wherein the second O-tRNA recognizes a selector codonthat is different from the selector codon recognized by the firstO-tRNA.
 8. The translation system of claim 1, wherein said systemcomprises a host cell comprising said first unnatural amino acid, saidfirst O-RS and said first O-tRNA.
 9. The translation system of claim 8,wherein said host cell is a eubacterial cell.
 10. The translation systemof claim 9, wherein said eubacterial cell is an E. coli cell.
 11. Thetranslation system of claim 8, wherein said host cell comprises apolynucleotide encoding said first O-RS.
 12. The translation system ofclaim 11, wherein said polynucleotide comprises a nucleotide sequenceset forth in SEQ ID NO: 5, 7 or
 9. 13. The translation system of claim8, wherein said host cell comprises a polynucleotide encoding said firstO-tRNA.
 14. The translation system of claim 1, wherein the O-RScomprises: a Glu or His at a position corresponding to position 65 ofSEQ ID NO: 6; an Ala or Gly at a position corresponding to position 67of SEQ ID NO: 6; a Phe, Asn or Lys at a position corresponding toposition 108 of SEQ ID NO: 6; a Gln or Ser at a position correspondingto position 109 of SEQ ID NO: 6; a GLN, Ser or Glu at a positioncorresponding to position 158 of SEQ ID NO: 6; or a Ser or Glu at aposition corresponding to position 162 of SEQ ID NO:
 6. 15. Thetranslation system of claim 14, wherein the O-RS is at least 95%identical to SEQ ID NO:
 6. 16. The translation system of claim 1,wherein the O-RS comprises: a Trp at a position corresponding toposition 32 of SEQ ID NO: 6; a Glu or His at a position corresponding toposition 65 of SEQ ID NO: 6; an Ala or Gly at a position correspondingto position 67 of SEQ ID NO: 6; a Gly at a position corresponding toposition 70 of SEQ ID NO: 6; a Phe, Asn or Lys at a positioncorresponding to position 108 of SEQ ID NO: 6; a Gln or Ser at aposition corresponding to position 109 of SEQ ID NO: 6; a GLN, Ser orGlu at a position corresponding to position 158 of SEQ ID NO: 6; and aSer or Glu at a position corresponding to position 162 of SEQ ID NO: 6.